[text, c.strings] Move text-related parts of [c.strings] to [text]

The text-related subclauses [cctype.syn], [cwctype.syn], [cwchar.syn],
[cuchar.syn], and [c.mb.wcs] are moved to a new subclause [text.c.strings].

Part of the C++26 clause restructuring (#5226, #5315).

Author: tkoeppe