keysym: add xkb_keysym_to_{lower,upper} to public API

keysym: add xkb_keysym_to_{lower,upper} to public API

These can be useful in some odd cases.

There is already an implementation (+ tests) for internal use, so all
that's needed is to export them.

If xkbcommon were to provide a way to convert a Unicode codepoint to a
keysym, this could have been implemented externally as follows:

    uint32_t codepoint = xkb_keysym_to_utf32(keysym);
    uint32_t upper_codepoint = my_unicode_library_to_upper(codepoint);
    xkb_keysym_t upper_keysym = theoretical_xkb_keysym_from_utf32(upper_codepoint);

However keysym -> codepoint is not injective so such a function is not
possible strictly speaking.

Signed-off-by: Ran Benita <ran234@gmail.com>