Backport #26745
Fixes #26548

This PR refactors the rendering of markup links. The old code uses
`strings.Replace` to change some urls while the new code uses more
context to decide which link should be generated.

The added tests should ensure the same output for the old and new
behaviour (besides the bug).

We may need to refactor the rendering a bit more to make it clear how
the different helper methods render the input string. There are lots of
options (resolve links / images / mentions / git hashes / emojis / ...)
but you don't really know what helper uses which options. For example,
we currently support images in the user description which should not be
allowed I think:

<details>
  <summary>Profile</summary>

https://try.gitea.io/KN4CK3R


![grafik](https://github.com/go-gitea/gitea/assets/1666336/109ae422-496d-4200-b52e-b3a528f553e5)

</details>

Replace #16455

Close #21803

Mixing different Gitea contexts together causes some problems:

1. Unable to respond proper content when error occurs, eg: Web should
respond HTML while API should respond JSON
2. Unclear dependency, eg: it's unclear when Context is used in
APIContext, which fields should be initialized, which methods are
necessary.


To make things clear, this PR introduces a Base context, it only
provides basic Req/Resp/Data features.

This PR mainly moves code. There are still many legacy problems and
TODOs in code, leave unrelated changes to future PRs.

- Update all tool dependencies to latest tag
- Remove unused errcheck, it is part of golangci-lint
- Include main.go in air
- Enable wastedassign again now that it's
[generics-compatible](https://github.com/golangci/golangci-lint/pull/3689)
- Restructured lint targets to new `lint-*` namespace

Remove `[repository.editor] PREVIEWABLE_FILE_MODES` setting that seemed
like it was intended to support this but did not work. Instead, whenever
viewing a file shows a preview, also have a Preview tab in the file
editor.

Add new `/markup` web and API endpoints with `comment`, `gfm`,
`markdown` and new `file` mode that uses a file path to determine the
renderer.

Remove `/markdown` web endpoint but keep the API for backwards and
GitHub compatibility.

## ⚠️ BREAKING ⚠️

The `[repository.editor] PREVIEWABLE_FILE_MODES` setting was removed.
This setting served no practical purpose and was not working correctly.
Instead a preview tab is always shown in the file editor when supported.

---------

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>