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.
7 jobs for !44 with topic/default/add-parser-as-entrypoint in 52 seconds (queued for 16 seconds)
latest detached
Status Name Job ID Coverage
  Tests
passed black #309185

00:00:33

passed check-manifest #309187

00:00:30

passed flake8 #309184

00:00:32

passed mypy #309186

00:00:38

passed py3 #309182

00:00:40

passed py3-from-forge #309183

00:00:50

 
  After Tests
passed trigger-cubicweb-pipeline