syntax: support grammar repository and include references.

* lib/zile/grammar/lua-bundle.lua (string.quoted.single.lua)
(string.quoted.double.lua): Use repository to reduce repetition
of escaped character patterns.
(string.quoted.single.lua, string.quoted.double.lua): Add
repository pattern for % placeholders in format strings.
* lib/zile/bundle.lua (compile_pattern): Factored out from the
inner loop of compile_patterns.
(load_grammar): Use compile_patterns on repository entries
too.
* lib/zile/syntax.lua (leftmost_match): Look-up include
references in the grammar repository, and perform matches against
any expressions or pattern list found there.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>