Support Flow this parameter annotations (#12234)

packages/babel-parser/test/fixtures/flow/this-annotation/function-declaration/input.js

@@ -0,0 +1,7 @@
+function foo (this : number, a : string, b) {}
+
+function bar (this : number) {}
+
+function baz (this : number, ...a) {}
+
+function qux<T> (this : T) {}