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/muesli/reflow/ansi/buffer.go

@@ -11,11 +11,16 @@ type Buffer struct {
 	bytes.Buffer
 }
 
-// PrintableRuneCount returns the amount of printable runes in the buffer.
-func (w Buffer) PrintableRuneCount() int {
+// PrintableRuneWidth returns the width of all printable runes in the buffer.
+func (w Buffer) PrintableRuneWidth() int {
+	return PrintableRuneWidth(w.String())
+}
+
+func PrintableRuneWidth(s string) int {
 	var n int
 	var ansi bool
-	for _, c := range w.String() {
+
+	for _, c := range s {
 		if c == '\x1B' {
 			// ANSI escape sequence
 			ansi = true

vendor/github.com/muesli/reflow/wordwrap/wordwrap.go

@@ -66,7 +66,7 @@ func (w *WordWrap) addSpace() {
 func (w *WordWrap) addWord() {
 	if w.word.Len() > 0 {
 		w.addSpace()
-		w.lineLen += w.word.PrintableRuneCount()
+		w.lineLen += w.word.PrintableRuneWidth()
 		w.buf.Write(w.word.Bytes())
 		w.word.Reset()
 	}
@@ -139,8 +139,8 @@ func (w *WordWrap) Write(b []byte) (int, error) {
 
 			// add a line break if the current word would exceed the line's
 			// character limit
-			if w.lineLen+w.space.Len()+w.word.PrintableRuneCount() > w.Limit &&
-				w.word.PrintableRuneCount() < w.Limit {
+			if w.lineLen+w.space.Len()+w.word.PrintableRuneWidth() > w.Limit &&
+				w.word.PrintableRuneWidth() < w.Limit {
 				w.addNewLine()
 			}
 		}