Problem: fg_indexed/bg_indexed were dropped from nvim_get_hl output due
to a wrong short_keys guard. HL_FG_INDEXED also wasn't cleared in
hl_blend_attrs, and HLATTRS_DICT_SIZE was too small.
Solution: Remove the short_keys guard, clear HL_FG_INDEXED in
hl_blend_attrs, bump HLATTRS_DICT_SIZE to 24, and clarify docs that
these flags mean rgb is an approximation of the cterm palette index.
01861c2f95