STL: Consider renaming <xmeow> internal headers

Many of our internal headers follow an <xmeow> naming convention (which is convenient as no Standard headers begin with x). Recently, we've been giving them extensions, like <xnode_handle.h> (mostly because the VS IDE needs to be updated when extensionless headers are added, so that they can be recognized as C++). We also have 3 internal headers of the form <ymeow.h>: <ymath.h>, <yvals.h>, and <yvals_core.h>. We have 1 internal header named <__msvc_all_public_headers.hpp> (which is for test purposes, not product code).

It's relatively uncommon, but we've noticed users occasionally including these internal headers (which are undocumented and unsupported; we frequently change their contents, and occasionally rename/delete the headers). We should consider changing this convention, but we need to decide on a consistent naming pattern (and probably choose better names for many of the headers; e.g. <xtr1common> is confusing).

Note: This will break user code, including the Windows codebase.

Ideas:

• <__msvc_internal_meow.hpp>
• <__msvc_stl_internal_meow.hpp>
• <__stl_internal_meow.hpp>

Review the listed internal headers, including the xmeow and ymeow.h naming patterns and __msvc_all_public_headers.hpp, then examine the proposed naming conventions and Windows codebase impact. Done means selecting a consistent replacement convention and identifying the headers and user-facing breakage it would affect.