Add support for a neovim-style extends query hierarchy.

[ctdunc]

Is your feature request related to a problem? Please describe.
I want to allow users to pass settings like "do/don't capitalize keywords" which would currently require me to maintain multiple query files where the content is largely the same.

Describe the solution you'd like
I would like to have some order in which multiple query files are loaded from different locations. e.g. the default ships at /usr/share/topiary/queries/lang.scm, but users can override it at .config/topiary/queries/lang.scm.

neovim added an ;extends modeline to their tree-sitter queries, which when present indicates that neovim should just append the text of the file to any previously loaded queries. See their very simple implementation

This works fine. In my SQL example, I tried out the query file (all the whitespace stuff omitted)

([
  (keyword_select) 
  (keyword_from)
  (keyword_where)
]) @upper_case
(keyword_select) @lower_case

which produces the following output:

select id
  ,column_a
  ,column_b
FROM foo
WHERE id = 0
;

from the input

sElEcT id, column_a, column_b FRoM foo WHeRE id = 0;

Describe alternatives you've considered
Shipping a grammar file for every possible permutation of settings a user could want for SQL. This is 👎