Make all file access 64-bit (uint64_t)

This changes the types of a big number of variables.

General rules:
- Using `uint64_t` in general. We also considered `int64_t` but eventually
  settled on keeping it unsigned, which is also closer to what one would expect
  with `size_t`/`off_t`.
- We only keep `int64_t` for `seek_end` (takes a negative offset from the end)
  and for the `Variant` bindings, since `Variant::INT` is `int64_t`. This means
  we only need to guard against passing negative values in `core_bind.cpp`.
- Using `uint32_t` integers for concepts not needing such a huge range, like
  pages, blocks, etc.

In addition:
- Improve usage of integer types in some related places; namely, `DirAccess`,
  core binds.

Note:
- On Windows, `_ftelli64` reports invalid values when using 32-bit MinGW with
  version < 8.0. This was an upstream bug fixed in 8.0. It breaks support for
  big files on 32-bit Windows builds made with that toolchain. We might add a
  workaround.

Fixes #44363.
Fixes godotengine/godot-proposals#400.

Co-authored-by: Rémi Verschelde <rverschelde@gmail.com>

modules/gdscript/gdscript.cpp

@@ -771,10 +771,10 @@ Error GDScript::load_source_code(const String &p_path) {
 		ERR_FAIL_COND_V(err, err);
 	}
 
-	int len = f->get_len();
+	uint64_t len = f->get_len();
 	sourcef.resize(len + 1);
 	PoolVector<uint8_t>::Write w = sourcef.write();
-	int r = f->get_buffer(w.ptr(), len);
+	uint64_t r = f->get_buffer(w.ptr(), len);
 	f->close();
 	memdelete(f);
 	ERR_FAIL_COND_V(r != len, ERR_CANT_OPEN);