Avoid fonts with incomplete coverage of MATHEMATICAL chars

Avoid fonts with incomplete coverage of MATHEMATICAL chars

* lisp/international/fontset.el (setup-default-fontset): Include both
ends of each sub-range in 'script-representative-chars' (bug#51495).

Some fonts include glyphs for MATHEMATICAL SCRIPT CAPITAL codepoints
(#x1D49C-#x1D4B5) but not for MATHEMATICAL SCRIPT SMALL codepoints
(#x1D4B6-#x1D4CF), see e.g. KpMath[1].

'script-representative-chars' must thus include both CAPITAL and SMALL
codepoints to ensure that we filter those fonts out.

Bug#51495; discussed in help-gnu-emacs[2].

[1] https://mirrors.ctan.org/fonts/kpfonts-otf/doc/unimath-kpfonts.pdf

[2] <878s02u5m6.fsf@gmail.com>
    https://lists.gnu.org/archive/html/help-gnu-emacs/2021-09/msg00150.html