[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/keywords.cpp

@@ -126,6 +126,7 @@ void define_keywords() {
       .add_keyword("builtin", Kw::_Builtin)
       .add_keyword("auto_apply", Kw::_AutoApply)
       .add_keyword("method_id", Kw::_MethodId)
+      .add_keyword("get", Kw::_Get)
       .add_keyword("operator", Kw::_Operator)
       .add_keyword("infix", Kw::_Infix)
       .add_keyword("infixl", Kw::_Infixl)