Expose GodotString formatting facility

[mivort]

This PR exposes godot_string_format API method to make built-in Godot string formatting available in GDNative API. It also adds a basic formatting in-engine test case.

[Bromeon]

Thanks! 🙂

With the suggested API, we don't support the placeholder argument, but I think that one is used so rarely that it should be OK. We can always add an "overload" later.

Could you maybe add an assert_eq! in the test that makes sure that the old string is unchanged? Just to be sure, and also for expressing the expected behavior.

Also, since the behavior is a bit special (see here) and format() accepts both arrays and dictionaries, do you think it makes sense to document the method with 2 examples, one for array and one for dictionary? Maybe also add a quick array test.

[mivort]

Do you think it makes sense to document the method with 2 examples, one for array and one for dictionary? Maybe also add a quick array test.

Sure, documentation always may come in handy :) I've added examples for index-style array values and dictionary, and also added test assertions for original string and array.

[Bromeon]

Combined the commits and changed ```ignore to ```no_run, so we can run compile-time doc tests on the examples. Retained author information with signed-off-by.

Thanks again!
bors r+

[bors]

Build succeeded:

• full-ci