[feature] enable footnote extension for markdown parser (#4251)

Signed-off-by: nicole mikołajczyk <git@mkljczk.pl>

# Description

This enables footnote support which is mentioned as allowed in the [user guide](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/docs/user_guide/posts.md#input-types). The regression was reported on Matrix.

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [sort of] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [ ] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [x] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4251
Co-authored-by: nicole mikołajczyk <git@mkljczk.pl>
Co-committed-by: nicole mikołajczyk <git@mkljczk.pl>

internal/text/markdown.go

@@ -141,6 +141,7 @@ func (f *Formatter) fromMarkdown(
 			extension.NewLinkify(
 				extension.WithLinkifyURLRegexp(regexes.URLLike),
 			),
+			extension.Footnote,
 			extension.Strikethrough,
 		),
 	)

internal/text/markdown_test.go

@@ -61,7 +61,7 @@ const (
 	mdCodeBlockWithNewlines            = "some code coming up\n\n```\n\n\n\n```\nthat was some code"
 	mdCodeBlockWithNewlinesExpected    = "<p>some code coming up</p><pre><code>\n\n\n</code></pre><p>that was some code</p>"
 	mdWithFootnote                     = "fox mulder,fbi.[^1]\n\n[^1]: federated bureau of investigation"
-	mdWithFootnoteExpected             = "<p>fox mulder,fbi.[^1]</p><p>[^1]: federated bureau of investigation</p>"
+	mdWithFootnoteExpected             = "<p>fox mulder,fbi.<sup id=\"fnref:1\"><a class=\"footnote-ref\">1</a></sup></p><div><hr><ol><li id=\"fn:1\"><p>federated bureau of investigation <a class=\"footnote-backref\">↩︎</a></p></li></ol></div>"
 	mdWithBlockQuote                   = "get ready, there's a block quote coming:\n\n>line1\n>line2\n>\n>line3\n\n"
 	mdWithBlockQuoteExpected           = "<p>get ready, there's a block quote coming:</p><blockquote><p>line1<br>line2</p><p>line3</p></blockquote>"
 	mdHashtagAndCodeBlock              = "#Hashtag\n\n```\n#Hashtag\n```"