Tweaks to `gdextension_interface.json` format

[dsnopek]

Fixes the non-documentation parts of https://github.com/godotengine/godot/issues/113732

This makes the following tweaks:

• Renames the "const" field to "is_const" to match a similarly named field in extension_api.json
• The "uninitialized" handles now refer to their "parent" handle and have an "is_uninitialized": true flag
• Enums that are bitfields have the "is_bitfield": true flag
• All instances of int are replaced with int32_t and int is removed as a valid type. Now all int types must have a size going forward!

[dsnopek]

I've just changed the boolean fields to start with is_* to match similarly named fields in extension_api.json (based on @Bromeon's suggest here)

[akien-mga]

Thanks!