parser: check function argument mutable syntax

vlib/v/checker/tests/mut_args_warning.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/mut_args_warning.v:1:8: warning: use `mut f Foo` instead of `f mut Foo`
+    1 | fn f(x mut []int) { x[0] = 1 }
+      |        ~~~
+    2 | fn main() {
+    3 |     mut x := [0]

vlib/v/checker/tests/mut_args_warning.vv

@@ -0,0 +1,5 @@
+fn f(x mut []int) { x[0] = 1 }
+fn main() {
+	mut x := [0]
+	f(mut x)
+}

vlib/v/parser/fn.v

@@ -415,6 +415,7 @@ fn (mut p Parser) fn_args() ([]table.Arg, bool) {
 			}
 			if p.tok.kind == .key_mut {
 				// TODO remove old syntax
+				p.warn_with_pos('use `mut f Foo` instead of `f mut Foo`', p.tok.position())
 				is_mut = true
 			}
 			if p.tok.kind == .ellipsis {