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/yuin/goldmark/parser/raw_html.go

@@ -2,10 +2,11 @@ package parser
 
 import (
 	"bytes"
+	"regexp"
+
 	"github.com/yuin/goldmark/ast"
 	"github.com/yuin/goldmark/text"
 	"github.com/yuin/goldmark/util"
-	"regexp"
 )
 
 type rawHTMLParser struct {
@@ -67,8 +68,6 @@ func (s *rawHTMLParser) parseSingleLineRegexp(reg *regexp.Regexp, block text.Rea
 	return node
 }
 
-var dummyMatch = [][]byte{}
-
 func (s *rawHTMLParser) parseMultiLineRegexp(reg *regexp.Regexp, block text.Reader, pc Context) ast.Node {
 	sline, ssegment := block.Position()
 	if block.Match(reg) {
@@ -102,7 +101,3 @@ func (s *rawHTMLParser) parseMultiLineRegexp(reg *regexp.Regexp, block text.Read
 	}
 	return nil
 }
-
-func (s *rawHTMLParser) CloseBlock(parent ast.Node, pc Context) {
-	// nothing to do
-}