refactor(tests): lint decorations_spec, float_spec, multigrid_spec #33274

Problem:
decorations_spec.lua, float_spec.lua, multigrid_spec.lua are not
auto-formatted.

Solution:
Add a special `formatlua2` cmake target, which invokes `stylua` with
an alternative `.stylua2.toml` config.

src/gen/luacats_grammar.lua

@@ -170,6 +170,7 @@ local ty_name = Cg(ty_ident, 'name')
 local opt_parent = opt(colon * Cg(ty_ident, 'parent'))
 local lname = (ident + ellipsis) * opt(P('?'))
 
+-- stylua: ignore
 local grammar = P {
   rep1(P('@') * (v.ats + v.ext_ats)),