Add --walker-path-sep=CHAR to use a different path separator

This is needed when you run a Windows binary on WSL or zsh on Windows
where forward slashes are expected.

  export FZF_DEFAULT_OPTS='--walker-path-sep /'

Close #3859

src/options.go

@@ -147,6 +147,7 @@ Usage: fzf [options]
     --walker-root=DIR       Root directory from which to start walker (default: .)
     --walker-skip=DIRS      Comma-separated list of directory names to skip
                             (default: .git,node_modules)
+    --walker-path-sep=CHAR  Path separator to use (default: / on Unix, \ on Windows)
 
   Shell integration
     --bash                  Print script to set up Bash shell integration
@@ -489,6 +490,7 @@ type Options struct {
 	WalkerOpts   walkerOpts
 	WalkerRoot   string
 	WalkerSkip   []string
+	WalkerSep    byte
 	Version      bool
 	Help         bool
 	CPUProfile   string
@@ -592,6 +594,7 @@ func defaultOptions() *Options {
 		WalkerOpts:   walkerOpts{file: true, hidden: true, follow: true},
 		WalkerRoot:   ".",
 		WalkerSkip:   []string{".git", "node_modules"},
+		WalkerSep:    os.PathSeparator,
 		Help:         false,
 		Version:      false}
 }
@@ -1904,6 +1907,14 @@ func parseMarkerMultiLine(str string) (*[3]string, error) {
 	return &result, nil
 }
 
+func parsePathSep(str string) (byte, error) {
+	bytes := []byte(str)
+	if len(bytes) != 1 {
+		return os.PathSeparator, errors.New("invalid path separator (expected: single-byte character)")
+	}
+	return bytes[0], nil
+}
+
 func parseOptions(index *int, opts *Options, allArgs []string) error {
 	var err error
 	var historyMax int
@@ -2479,6 +2490,14 @@ func parseOptions(index *int, opts *Options, allArgs []string) error {
 				return err
 			}
 			opts.WalkerSkip = filterNonEmpty(strings.Split(str, ","))
+		case "--walker-path-sep":
+			str, err := nextString(allArgs, &i, "path separator required")
+			if err != nil {
+				return err
+			}
+			if opts.WalkerSep, err = parsePathSep(str); err != nil {
+				return err
+			}
 		case "--profile-cpu":
 			if opts.CPUProfile, err = nextString(allArgs, &i, "file path required: cpu"); err != nil {
 				return err
@@ -2666,6 +2685,10 @@ func parseOptions(index *int, opts *Options, allArgs []string) error {
 				opts.WalkerRoot = value
 			} else if match, value := optString(arg, "--walker-skip="); match {
 				opts.WalkerSkip = filterNonEmpty(strings.Split(value, ","))
+			} else if match, value := optString(arg, "--walker-path-sep="); match {
+				if opts.WalkerSep, err = parsePathSep(value); err != nil {
+					return err
+				}
 			} else if match, value := optString(arg, "--hscroll-off="); match {
 				if opts.HscrollOff, err = atoi(value); err != nil {
 					return err