Unify TextServer built-in module and GDExtension code.

[bruvzg]

Allows build the same TextServer code as a GDExtension or built-in module.

• Adds String operator + char32_t.
• Exposes String::num_int64 and String::num_uint64 methods to scripting.
• Exposes Image::create_from_data and Image::create_empty to C++.
• Makes TextServer interface GDExtension compatible.
• Adds SCons scripts to build TextServer implementation as GDExtension.
• Adds a TextServerDummy, which is not capable of loading fonts and rendering text, but allows building Godot (including editor builds) without real text servers and FreeType.

GDExtension modules depends on godotengine/godot-cpp#701, and require extension_api.json update to build.

[bruvzg]

Some binary size stats:

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Editor release-debug build (ARM64 macOS build):

With both Advanced and Fallback text servers: Unstripped 159 MB, stripped 118 MB.
Without any text servers, and FreeType: Unstripped 147 MB, stripped 107 MB.

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Export template build release (ARM64 macOS build, with LTO):

With both Advanced and Fallback text servers: stripped 70 MB.
Without any text servers, and FreeType: stripped 63 MB.

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GDExtension modules (works with both exported project and editor, Universal ARM64+x86_64 macOS build):

Advanced - Unstripped debug 32 MB, stripped debug 19 MB, unstripped release 20 MB, stripped release 18 MB.
Fallback - Unstripped debug 10 MB, stripped debug 6 MB, unstripped release 6 MB, stripped release 5 MB.

[akien-mga]

I guess we don't really need the editor_module_list logic now.

That being said, we could decide to make both regex and svg modules mandatory for the editor so I guess we can keep it for later use.

[akien-mga]

Thanks!