refactor: Add module entrypoint for parsing rql strings

Move the old code within `parser_main.py` to `parser/__main__.py`
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 __main__.py will be executed and the string "Any 1+1;"
will be parsed.
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