Detect markdown line width, resolve relative URLs (#332)

~~this is semi-blocked by https://github.com/charmbracelet/glamour/pull/96, but behaviour isn't really worse than the previous behaviour (most links work, some are still broken)~~

#### testcase for link resolver
```
tea pr 332
tea checkout 332 && make install && tea pr 332
```

- [rel](./332)
- [abs](/gitea/tea/pulls/332)
- [full](https://gitea.com/gitea/tea/pulls/332)

Co-authored-by: Norwin Roosen <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/332
Reviewed-by: 6543 <6543@obermui.de>
Reviewed-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

vendor/github.com/charmbracelet/glamour/ansi/image.go

@@ -25,7 +25,7 @@ func (e *ImageElement) Render(w io.Writer, ctx RenderContext) error {
 	}
 	if len(e.URL) > 0 {
 		el := &BaseElement{
-			Token:  resolveRelativeURL(e.BaseURL, e.URL),
+			Token:  resolveURL(e.BaseURL, e.URL),
 			Prefix: " ",
 			Style:  ctx.options.Styles.Image,
 		}