font{,/{opentype,gofont}},text: [API] drop monospace font metadata

In the general case, it isn't possible for us to efficiently find system fonts that
are monospace. Fonts don't advertise being monospace frequently, so the only way to
reliably detect it is to check that all glyphs are the same width. This is expensive,
far too much so to be done on every system font when there may be thousands of them.

Other font resolution systems rely upon the user requesting fonts by their family name.
If you want a monospace font, ask for it by name or use a generic name like 'monospace'.
This will be Gio's approach from here on out.

Existing code relying upon setting Variant="Mono" should instead set Typeface="Go Mono"
(for the Go font) or specify another monospace typeface. The generic face "monospace"
will search for one of a set of known monospace fonts that may be available on the system.

Similarly, smallcaps isn't well advertised and users should rely on requesting all-smallcaps
fonts by typeface. To get the Go smallcaps font, use Typeface="Go Smallcaps".

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

font/font.go

@@ -20,10 +20,11 @@ type Weight int
 
 // Font specify a particular typeface variant, style and weight.
 type Font struct {
+	// Typeface specifies the name of the family of faces.
 	Typeface Typeface
-	Variant  Variant
-	Style    Style
-	// Weight is the text weight. If zero, Normal is used instead.
+	// Style specifies the kind of text style.
+	Style Style
+	// Weight is the text weight.
 	Weight Weight
 }
 
@@ -37,9 +38,6 @@ type Face interface {
 // string denotes the default typeface.
 type Typeface string
 
-// Variant denotes a typeface variant such as "Mono" or "Smallcaps".
-type Variant string
-
 const (
 	Regular Style = iota
 	Italic