[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,
 		),
 	)