msgconv: port most of attachment and text message bridging

* Created a separate discordid package to avoid import cycles.
* Implemented attachment bridging. We still need to implement direct
  media, but this will do for now.
* Corrected how encrypted files (e.g. embed images and attachments) were
  bridged. Previously, the URL field would be empty.

Still a lot of missing pieces. Thoughts:

* Mentions to roles and custom emoji are not rendered properly. We need
  to maintain our own DB.
* We might not need the "attachments" leaf package anymore? It's just
  there to avoid an import cycle.

Bridging actual events (i.e. wiring up discordgo's event handlers) is
probably next.

pkg/msgconv/formatter.go

@@ -0,0 +1,99 @@
+// mautrix-discord - A Matrix-Discord puppeting bridge.
+// Copyright (C) 2024 Tulir Asokan
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package msgconv
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/extension"
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/util"
+	"maunium.net/go/mautrix/bridgev2"
+	"maunium.net/go/mautrix/format"
+	"maunium.net/go/mautrix/format/mdext"
+)
+
+// escapeFixer is a hacky partial fix for the difference in escaping markdown, used with escapeReplacement
+//
+// Discord allows escaping with just one backslash, e.g. \__a__,
+// but standard markdown requires both to be escaped (\_\_a__)
+var escapeFixer = regexp.MustCompile(`\\(__[^_]|\*\*[^*])`)
+
+func escapeReplacement(s string) string {
+	return s[:2] + `\` + s[2:]
+}
+
+// indentableParagraphParser is the default paragraph parser with CanAcceptIndentedLine.
+// Used when disabling CodeBlockParser (as disabling it without a replacement will make indented blocks disappear).
+type indentableParagraphParser struct {
+	parser.BlockParser
+}
+
+var defaultIndentableParagraphParser = &indentableParagraphParser{BlockParser: parser.NewParagraphParser()}
+
+func (b *indentableParagraphParser) CanAcceptIndentedLine() bool {
+	return true
+}
+
+var removeFeaturesExceptLinks = []any{
+	parser.NewListParser(), parser.NewListItemParser(), parser.NewHTMLBlockParser(), parser.NewRawHTMLParser(),
+	parser.NewSetextHeadingParser(), parser.NewThematicBreakParser(),
+	parser.NewCodeBlockParser(),
+}
+var removeFeaturesAndLinks = append(removeFeaturesExceptLinks, parser.NewLinkParser())
+var fixIndentedParagraphs = goldmark.WithParserOptions(parser.WithBlockParsers(util.Prioritized(defaultIndentableParagraphParser, 500)))
+var discordExtensions = goldmark.WithExtensions(extension.Strikethrough, mdext.SimpleSpoiler, mdext.DiscordUnderline, ExtDiscordEveryone, ExtDiscordTag)
+
+var discordRenderer = goldmark.New(
+	goldmark.WithParser(mdext.ParserWithoutFeatures(removeFeaturesAndLinks...)),
+	fixIndentedParagraphs, format.HTMLOptions, discordExtensions,
+)
+var discordRendererWithInlineLinks = goldmark.New(
+	goldmark.WithParser(mdext.ParserWithoutFeatures(removeFeaturesExceptLinks...)),
+	fixIndentedParagraphs, format.HTMLOptions, discordExtensions,
+)
+
+// renderDiscordMarkdownOnlyHTML converts Discord-flavored Markdown text to HTML.
+//
+// After conversion, if the text is surrounded by a single outermost paragraph
+// tag, it is unwrapped.
+func (mc *MessageConverter) renderDiscordMarkdownOnlyHTML(portal *bridgev2.Portal, text string, allowInlineLinks bool) string {
+	return format.UnwrapSingleParagraph(mc.renderDiscordMarkdownOnlyHTMLNoUnwrap(portal, text, allowInlineLinks))
+}
+
+// renderDiscordMarkdownOnlyHTMLNoUnwrap converts Discord-flavored Markdown text to HTML.
+func (mc *MessageConverter) renderDiscordMarkdownOnlyHTMLNoUnwrap(portal *bridgev2.Portal, text string, allowInlineLinks bool) string {
+	text = escapeFixer.ReplaceAllStringFunc(text, escapeReplacement)
+
+	var buf strings.Builder
+	ctx := parser.NewContext()
+	ctx.Set(parserContextPortal, portal)
+	renderer := discordRenderer
+	if allowInlineLinks {
+		renderer = discordRendererWithInlineLinks
+	}
+	err := renderer.Convert([]byte(text), &buf, parser.WithContext(ctx))
+	if err != nil {
+		panic(fmt.Errorf("markdown parser errored: %w", err))
+	}
+	return buf.String()
+}
+
+// TODO(skip): Stopping here for now. Continue at formatterContextPortalKey.