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/lines.go

@@ -3,13 +3,16 @@
 
 package markdown
 
+import "strings"
+
 type Line struct {
 	Range
 }
 
-func ParseLines(markdown string) (lines []Line) {
+func ParseLines(markdown string) []Line {
 	lineStartPosition := 0
 	isAfterCarriageReturn := false
+	lines := make([]Line, 0, strings.Count(markdown, "\n"))
 	for position, r := range markdown {
 		if r == '\n' {
 			lines = append(lines, Line{Range{lineStartPosition, position + 1}})
@@ -23,5 +26,5 @@ func ParseLines(markdown string) (lines []Line) {
 	if lineStartPosition < len(markdown) {
 		lines = append(lines, Line{Range{lineStartPosition, len(markdown)}})
 	}
-	return
+	return lines
 }