refactor: Add module entrypoint for parsing rql strings

Move the old code within `` to `parser/`
So that rql package can be run with the -m module flag.

When calling `$ python3 -m rql.parser "Any 1+1;"`, the main function
defined inside will be executed and the string "Any 1+1;"
will be parsed.
3 jobs for branch/default in 1 minute and 7 seconds (queued for 2 seconds)