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/util/util.go

@@ -37,6 +37,12 @@ func (b *CopyOnWriteBuffer) Write(value []byte) {
 	b.buffer = append(b.buffer, value...)
 }
 
+// WriteString writes given string to the buffer.
+// WriteString allocate new buffer and clears it at the first time.
+func (b *CopyOnWriteBuffer) WriteString(value string) {
+	b.Write(StringToReadOnlyBytes(value))
+}
+
 // Append appends given bytes to the buffer.
 // Append copy buffer at the first time.
 func (b *CopyOnWriteBuffer) Append(value []byte) {
@@ -49,6 +55,12 @@ func (b *CopyOnWriteBuffer) Append(value []byte) {
 	b.buffer = append(b.buffer, value...)
 }
 
+// AppendString appends given string to the buffer.
+// AppendString copy buffer at the first time.
+func (b *CopyOnWriteBuffer) AppendString(value string) {
+	b.Append(StringToReadOnlyBytes(value))
+}
+
 // WriteByte writes the given byte to the buffer.
 // WriteByte allocate new buffer and clears it at the first time.
 func (b *CopyOnWriteBuffer) WriteByte(c byte) {
@@ -804,7 +816,7 @@ func IsPunct(c byte) bool {
 	return punctTable[c] == 1
 }
 
-// IsPunct returns true if the given rune is a punctuation, otherwise false.
+// IsPunctRune returns true if the given rune is a punctuation, otherwise false.
 func IsPunctRune(r rune) bool {
 	return int32(r) <= 256 && IsPunct(byte(r)) || unicode.IsPunct(r)
 }
@@ -814,7 +826,7 @@ func IsSpace(c byte) bool {
 	return spaceTable[c] == 1
 }
 
-// IsSpace returns true if the given rune is a space, otherwise false.
+// IsSpaceRune returns true if the given rune is a space, otherwise false.
 func IsSpaceRune(r rune) bool {
 	return int32(r) <= 256 && IsSpace(byte(r)) || unicode.IsSpace(r)
 }