widget: expose text region resolution

This commit adds exported methods to both LabelState and Editor
allowing callers to locate the text regions representing a range
of runes. This can be used to build interactive subregions of text,
like (for instance) hyperlinks.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/index.go

@@ -307,6 +307,10 @@ type Region struct {
 	Baseline int
 }
 
+// region is identical to Region except that its coordinates are in document
+// space instead of a widget coordinate space.
+type region = Region
+
 // locate returns highlight regions covering the glyphs that represent the runes in
 // [startRune,endRune). If the rects parameter is non-nil, locate will use it to
 // return results instead of allocating, provided that there is enough capacity.