Add the .rbi file extension as a Ruby file type.

.rbi files are type definition files that are currently used by some
community type checkers, most notably Stripe's Sorbet,
and will be part of the main language in Ruby 3.

extensions/ruby/package.json

@@ -12,7 +12,7 @@
 	"contributes": {
 		"languages": [{
 			"id": "ruby",
-			"extensions": [ ".rb", ".rbx", ".rjs", ".gemspec", ".rake", ".ru", ".erb",".podspec" ],
+			"extensions": [ ".rb", ".rbx", ".rjs", ".gemspec", ".rake", ".ru", ".erb", ".podspec", ".rbi" ],
 			"filenames": [ "rakefile", "gemfile", "guardfile", "podfile", "capfile" ],
 			"aliases": [ "Ruby", "rb" ],
 			"firstLine": "^#!\\s*/.*\\bruby\\b",