MM-29981 - optimize markdown.Inspect (#16188)

* MM-29981 - optimize markdown.Inspect

- Cache regexp.MustCompile
- Reuse slice in MergeInlineText
- Remove pointer to slice in closeBlocks
- Pre-allocate slice in ParseLines
- Some more small cleanups

```
name       old time/op    new time/op    delta
Inspect-8    10.5µs ± 3%     6.6µs ± 1%  -37.59%  (p=0.000 n=10+7)

name       old alloc/op   new alloc/op   delta
Inspect-8    6.66kB ± 0%    3.22kB ± 0%  -51.62%  (p=0.000 n=10+9)

name       old allocs/op  new allocs/op  delta
Inspect-8       117 ± 0%        76 ± 0%  -35.04%  (p=0.000 n=10+10)
```

* fix lint

* remove ignore

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

utils/markdown/markdown.go

@@ -23,9 +23,8 @@ func isWhitespace(c rune) bool {
 	switch c {
 	case ' ', '\t', '\n', '\u000b', '\u000c', '\r':
 		return true
-	default:
-		return false
 	}
+	return false
 }
 
 func isWhitespaceByte(c byte) bool {