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/alecthomas/chroma/lexers/circular/phtml.go

@@ -0,0 +1,34 @@
+package circular
+
+import (
+	"strings"
+
+	. "github.com/alecthomas/chroma" // nolint
+	"github.com/alecthomas/chroma/lexers/h"
+	"github.com/alecthomas/chroma/lexers/internal"
+)
+
+// PHTML lexer is PHP in HTML.
+var PHTML = internal.Register(DelegatingLexer(h.HTML, MustNewLexer(
+	&Config{
+		Name:            "PHTML",
+		Aliases:         []string{"phtml"},
+		Filenames:       []string{"*.phtml"},
+		MimeTypes:       []string{"application/x-php", "application/x-httpd-php", "application/x-httpd-php3", "application/x-httpd-php4", "application/x-httpd-php5"},
+		DotAll:          true,
+		CaseInsensitive: true,
+		EnsureNL:        true,
+	},
+	Rules{
+		"root": {
+			{`<\?(php)?`, CommentPreproc, Push("php")},
+			{`[^<]+`, Other, nil},
+			{`<`, Other, nil},
+		},
+	}.Merge(phpCommonRules),
+).SetAnalyser(func(text string) float32 {
+	if strings.Contains(text, "<?php") {
+		return 0.5
+	}
+	return 0.0
+})))