Missing `GodotString` methods

[chitoyuu]

Created during survey of commented code (#377).

A large number of highly specific methods in the "Helper" class are still missing from the GodotString API. Some (non-exhaustive) examples are:

• godot_string_format_with_custom_placeholder
• godot_string_lpad_with_custom_character
• godot_string_bigrams
• godot_string_casecmp_to
• godot_string_nocasecmp_to
• godot_string_naturalnocasecmp_to
• ...etc

Wrappers for these are in principle nice to have for the sake of completeness, but history of actual usage has shown that people usually prefer to use the native String type which has much better interoperability with the rest of the ecosystem.