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/svg/image_loader_svg.cpp

@@ -140,7 +140,7 @@ Error ImageLoaderSVG::create_image_from_string(Ref<Image> p_image, const char *p
 }
 
 Error ImageLoaderSVG::load_image(Ref<Image> p_image, FileAccess *f, bool p_force_linear, float p_scale) {
-	uint32_t size = f->get_len();
+	uint64_t size = f->get_len();
 	Vector<uint8_t> src_image;
 	src_image.resize(size + 1);
 	uint8_t *src_w = src_image.ptrw();