* 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
        ^

/cc @tessus

To make it consistent with select-all and toggle-all.

Close #1364

https://github.com/junegunn/vim-plug/pull/891

Close #1718
Related #688

If present the contents of the selection will be placed in a temporary file,
and the filename will be placed into the string instead.

Close #1708

https://github.com/junegunn/vim-plug/pull/888

By @gh4w and @janlazo

See https://github.com/junegunn/vim-plug/commit/68b31a4a66ec945ff299db25a8a6382cd48edf14

Related #1704

Fix https://github.com/junegunn/fzf.vim/issues/855

While we can attach `_fzf_path_completion` or `_fzf_dir_completion` to
any command using the standard bash complete command, the functionality
of the existing completion function is lost.

Use _fzf_setup_completion if you want to extend the existing function
with fuzzy completion instead of completely replacing it.

e.g. _fzf_setup_completion path kubectl

- Replaced time.Now().Sub() with time.Since()
- Replaced unnecessary string/byte slice conversions
- Removed obsolete return and value assignment in range loop

Perl was used to remove the trailing newline character, but fzf already
has --print0 to use null character as terminators, and fish read -z is
expecting null character as terminators. There is no reason to depend on
perl if --print0 is passed to fzf invocation.

(i.e. spawn xterm)

The section is no longer relevant since (G)Vim 8 or above has builtin
terminal emulator.

Fix #1592

A newer version of the sys module is needed for pull request #1341

[bash-completion] Fix custom completion with dynamic loader enabled for non-standard command names (#1564)

Related to #1170.

Fix the solution for commands with non-standard names
where `$cmd` and `$orig_cmd` differ. e.g. `s.foo` -> `s_foo`

Close #1554