[FunC] Deprecate method_id specifier, introduce get keyword

`get` keyword behaves exactly like `method_id` (auto-calc hash),
but it's placed on the left, similar to Tact: `get T name()`.

`method_id(n)` is still valid, considering it can't be invoked by name,
since a client will compute another hash.
It's supposed it will be still used in tests and in low-level code
(not to be called externally, but to be called after replacing c3).

`get(hash)` is invalid, this keyword does not accept anything.

crypto/func/auto-tests/tests/invalid-get-method-1.fc

@@ -0,0 +1,8 @@
+get (int, int) hello(int x, int y) method_id(123) {
+    return (x, y);
+}
+
+{-
+@compilation_should_fail
+@stderr both `get` and `method_id` are not allowed
+-}