Add flat-square action badge style (#34062)

Adds the `flat-square` style to action badges. Styles can be selected by
adding `?style=<style>` to the badge endpoint. If no style query is
given, or if the query is invalid, the style defaults to `flat`.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/badge/badge_glyph_width.go

@@ -3,8 +3,6 @@
 
 package badge
 
-import "sync"
-
 // DejaVuGlyphWidthData is generated by `sfnt.Face.GlyphAdvance(nil, <rune>, 11, font.HintingNone)` with DejaVu Sans
 // v2.37 (https://github.com/dejavu-fonts/dejavu-fonts/releases/download/version_2_37/dejavu-sans-ttf-2.37.zip).
 //
@@ -13,7 +11,7 @@ import "sync"
 //
 // A devtest page "/devtest/badge-actions-svg" could be used to check the rendered images.
 
-var DejaVuGlyphWidthData = sync.OnceValue(func() map[rune]uint8 {
+func dejaVuGlyphWidthDataFunc() map[rune]uint8 {
 	return map[rune]uint8{
 		32:  3,
 		33:  4,
@@ -205,4 +203,4 @@ var DejaVuGlyphWidthData = sync.OnceValue(func() map[rune]uint8 {
 		254: 7,
 		255: 7,
 	}
-})
+}