[PATCH] [Bug #21629] Enable `nonstring` attribute on clang 21

	[PATCH] [Bug #21629] Initialize `struct RString`

	[PATCH] [Bug #21629] Initialize `struct RArray`

	[Bug #20322] Fix rb_enc_interned_str_cstr null encoding

	The documentation for `rb_enc_interned_str_cstr` notes that `enc` can be
	a null pointer, but this currently causes a segmentation fault when
	trying to autoload the encoding. This commit fixes the issue by checking
	for NULL before calling `rb_enc_autoload`.

* Unify length field for embedded and heap strings

The length field is of the same type and position in RString for both
embedded and heap allocated strings, so we can unify it.

* Remove RSTRING_EMBED_LEN

Remove !USE_RVARGC code

[Feature #19579]

The Variable Width Allocation feature was turned on by default in Ruby
3.2. Since then, we haven't received bug reports or backports to the
non-Variable Width Allocation code paths, so we assume that nobody is
using it. We also don't plan on maintaining the non-Variable Width
Allocation code, so we are going to remove it.

Split `PACKED_STRUCT` and `PACKED_STRUCT_UNALIGNED` macros into the
macros bellow:
* `RBIMPL_ATTR_PACKED_STRUCT_BEGIN`
* `RBIMPL_ATTR_PACKED_STRUCT_END`
* `RBIMPL_ATTR_PACKED_STRUCT_UNALIGNED_BEGIN`
* `RBIMPL_ATTR_PACKED_STRUCT_UNALIGNED_END`