Improvements to code quality and readability (#1737)

* Remove 1 unused field and 3 unused functions

unused elements fount by running
golangci-lint run --disable-all --enable unused

src/result.go:19:2: field `index` is unused (unused)
        index  int32
        ^
src/tui/light.go:716:23: func `(*LightWindow).stderr` is unused (unused)
func (w *LightWindow) stderr(str string) {
                      ^
src/terminal.go:1015:6: func `numLinesMax` is unused (unused)
func numLinesMax(str string, max int) int {
     ^
src/tui/tui.go:167:20: func `ColorPair.is24` is unused (unused)
func (p ColorPair) is24() bool {
                   ^

* Address warnings from "gosimple" linter

src/options.go:389:83: S1003: should use strings.Contains(str, ",,,") instead (gosimple)
        if str == "," || strings.HasPrefix(str, ",,") || strings.HasSuffix(str, ",,") || strings.Index(str, ",,,") >= 0 {
                                                                                         ^
src/options.go:630:18: S1007: should use raw string (`...`) with regexp.MustCompile to avoid having to escape twice (gosimple)
        executeRegexp = regexp.MustCompile(
                        ^
src/terminal.go:29:16: S1007: should use raw string (`...`) with regexp.MustCompile to avoid having to escape twice (gosimple)
        placeholder = regexp.MustCompile("\\\\?(?:{[+sf]*[0-9,-.]*}|{q}|{\\+?f?nf?})")
                      ^
src/terminal_test.go:92:10: S1007: should use raw string (`...`) with regexp.MustCompile to avoid having to escape twice (gosimple)
        regex = regexp.MustCompile("\\w+")
                ^

* Address warnings from "staticcheck" linter

src/algo/algo.go:374:2: SA4006: this value of `offset32` is never used (staticcheck)
        offset32, T := alloc32(offset32, slab, N)
        ^
src/algo/algo.go:456:2: SA4006: this value of `offset16` is never used (staticcheck)
        offset16, C := alloc16(offset16, slab, width*M)
        ^
src/tui/tui.go:119:2: SA9004: only the first constant in this group has an explicit type (staticcheck)
        colUndefined Color = -2
        ^

1 files changed, 1 insertions, 14 deletions

diff --git a/src/terminal.go b/src/terminal.go
index 3e5683c6..3c656cce 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -26,7 +26,7 @@ var placeholder *regexp.Regexp
 var activeTempFiles []string
 
 func init() {
-	placeholder = regexp.MustCompile("\\\\?(?:{[+sf]*[0-9,-.]*}|{q}|{\\+?f?nf?})")
+	placeholder = regexp.MustCompile(`\\?(?:{[+sf]*[0-9,-.]*}|{q}|{\+?f?nf?})`)
 	activeTempFiles = []string{}
 }
 
@@ -1012,19 +1012,6 @@ func (t *Terminal) printHighlighted(result Result, attr tui.Attr, col1 tui.Color
 	return displayWidth
 }
 
-func numLinesMax(str string, max int) int {
-	lines := 0
-	for lines < max {
-		idx := strings.Index(str, "\n")
-		if idx < 0 {
-			break
-		}
-		str = str[idx+1:]
-		lines++
-	}
-	return lines
-}
-
 func (t *Terminal) printPreview() {
 	if !t.hasPreviewWindow() {
 		return