Add many GString/StringName methods

[Bromeon]

Populates GString + StringName with the majority of generated methods from Godot.

I made multiple API design choices to make string operations more idiomatic in Rust:

• Option::None instead of -1 for "not found".
• Builder pattern for split + find, instead of proliferation of find/findn/rfind/rfindn and x2 with _from index.
• Use usize instead of i64 where integers are unsigned offsets/indices.
• Use std::ops::RangeBounds instead of inconsistent from, (from, to), (from, len) conventions used in Godot.
• Use char instead of i64 when a Unicode code point is required.
• Use std::cmp::Ordering when the result is -1, 0 or 1 depending on ordering relation.
• Add a with_* overload for rarely used alternatives (format + format_with_placeholder).
• Rename in case of ambiguity (match -> match_glob).

Added unit tests for most things.

For the [natural|file][no]casecmp_to methods, I currently left the original methods, but technically they could use a builder as well. Might change in the future...

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-980

[lilizoey]

I think it would be nice if we could like have some feature where we can change at least the return value of some auto-generated functions.

Like maybe is_builtin_method_exposed could return an Option<ExposeOptions> or something, and ExposeOptions is an enum with variants like

enum ExposeOptions {
    Default,
    Ordering, // -1 = Less, 0 = Equal, 1 = Greater
    Option, 
    Usize, 
}

Which will have the code attempt to change the return value to that type? idk

maybe at least group the functions somehow for now by which should remain unchanged and which should have their signature modified so we can more easily change it after implementing something like the above.

Otherwise i think this is pretty good.

[lilizoey]

Suggested change

	/// _Godot equivalent: `length`_
	pub fn len(&self) -> usize {
	/// _Godot equivalent: `length`_
	#[doc(alias = "length")]
	pub fn len(&self) -> usize {

[lilizoey]

Should these functions return a Ordering? They do return -1, 0, 1 corresponding to less/equal/greater. Same with StringName below

[lilizoey]

There are a some functions like these that can fail that should maybe return Option<T> instead, though get_slice actually is a bit weird in that there are two failure cases that have different behavior.

[Bromeon]

So, I added a ton of methods, including several manually redefined ones. See initial post for API design.