query_language: Setup tree-sitter and grammar.js

This revision defines the grammar for the search query language and
prepares swh.search for a smoother development of the grammar.

The parsers generated from the proposed grammar serve two different purposes:

• Translation of search queries into elasticsearch DSL in swh.search (or any

other search backend that we may use in the future)

• Autocompletion of the queries in the swh.web (Archive UI)

tree-sitter has been selected for the task because it has bindings for
python (swh.search) as well as wasm (swh.web).