fix(api): cterm type in highlight keyset #37802

Problem: cterm field in Dict(highlight) is declared as Union(Integer, String)
but it actually expects a Dict(highlight_cterm).

Solution: change cterm type to DictAs(highlight__cterm) and simplify the
handling in dict2hlattrs since type validation and empty array compat are
already handled by api_dict_to_keydict.

runtime/lua/vim/_meta/api_keysets.lua

@@ -308,7 +308,7 @@ error('Cannot require a meta file')
 --- @field altfont? boolean
 --- @field nocombine? boolean
 --- @field default? boolean
---- @field cterm? integer|string
+--- @field cterm? vim.api.keyset.highlight_cterm
 --- @field foreground? integer|string
 --- @field fg? integer|string
 --- @field background? integer|string