feat(parser): add optional display-name to layer (#20)

Add support for an optional label property in layers
which can match zero or once.

In ZMK each layer can be assigned a display-name. One use case for
the label is that it can be shown in display widgets.

See zmkfirmware/zmk#551

pkg/keymap/parser.go

@@ -84,6 +84,7 @@ type Layer struct {
 	Pos lexer.Position
 
 	Name           string      `parser:"@Ident '{'"`
+	DisplayName    string      `parser:"('display-name' '=' @String';')?"`
 	Bindings       []*Behavior `parser:"'bindings' '=' '<'@@+'>'';'"`
 	SensorBindings []*Behavior `parser:"('sensor''-''bindings' '=' '<'@@+'>'';')?"`
 	EndBrace       string      `parser:" '}'';'"`