pkg/elvdoc: Fix the handling of options with spaces in the value.

Instead of using strings.Fields, use the actual Elvish parser to retrieve
options and arguments.

Among probably others, this fixes the elvdoc of "echo", whose usage previously
read (note the bogus $ in the default value of &sep):

echo &sep=' $' $value...

pkg/elvdoc/elvdoc.go

@@ -6,6 +6,8 @@ import (
 	"io"
 	"regexp"
 	"strings"
+
+	"src.elv.sh/pkg/parse"
 )
 
 // Docs records doc comments.
@@ -90,7 +92,7 @@ func fnUsage(name, sig string) string {
 	var sb strings.Builder
 	sb.WriteString("```elvish\n")
 	sb.WriteString(name)
-	for _, field := range strings.Fields(sig) {
+	for _, field := range sigFields(sig) {
 		sb.WriteByte(' ')
 		if strings.HasPrefix(field, "&") {
 			sb.WriteString(field)
@@ -104,6 +106,23 @@ func fnUsage(name, sig string) string {
 	return sb.String()
 }
 
+func sigFields(sig string) []string {
+	pn := &parse.Primary{}
+	// TODO: Handle error
+	parse.ParseAs(parse.Source{Code: "{|" + sig + "|}"}, pn, parse.Config{})
+	var fields []string
+	for _, n := range parse.Children(pn) {
+		if _, isSep := n.(*parse.Sep); isSep {
+			continue
+		}
+		s := strings.TrimSpace(parse.SourceText(n))
+		if s != "" {
+			fields = append(fields, s)
+		}
+	}
+	return fields
+}
+
 type docBlock struct {
 	id           string
 	lines        []string

pkg/elvdoc/elvdoc_test.go

@@ -67,6 +67,22 @@ var extractTests = []struct {
 			},
 		},
 	},
+	{
+		name: "option with space",
+		text: dedent(`
+			fn add {|a b &k=' '| }
+			`),
+		wantFns: []Entry{
+			{
+				Name: "add",
+				Content: dedent(tildeToBackquote(`
+						~~~elvish
+						add $a $b &k=' '
+						~~~
+						`)),
+			},
+		},
+	},
 	{
 		name: "fn with rest argument",
 		text: dedent(`