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-emoji/ast/emoji.go

@@ -0,0 +1,42 @@
+// Package ast defines AST nodes that represetns emoji extension's elements.
+package ast
+
+import (
+	"fmt"
+
+	"github.com/yuin/goldmark-emoji/definition"
+	gast "github.com/yuin/goldmark/ast"
+)
+
+// Emoji represents an inline emoji.
+type Emoji struct {
+	gast.BaseInline
+
+	ShortName []byte
+	Value     *definition.Emoji
+}
+
+// Dump implements Node.Dump.
+func (n *Emoji) Dump(source []byte, level int) {
+	m := map[string]string{
+		"ShortName": string(n.ShortName),
+		"Value":     fmt.Sprintf("%#v", n.Value),
+	}
+	gast.DumpHelper(n, source, level, m, nil)
+}
+
+// KindEmoji is a NodeKind of the emoji node.
+var KindEmoji = gast.NewNodeKind("Emoji")
+
+// Kind implements Node.Kind.
+func (n *Emoji) Kind() gast.NodeKind {
+	return KindEmoji
+}
+
+// NewEmoji returns a new Emoji node.
+func NewEmoji(shortName []byte, value *definition.Emoji) *Emoji {
+	return &Emoji{
+		ShortName: shortName,
+		Value:     value,
+	}
+}