add lark-based grammar

[dirk-thomas]

This is the fourth PR integrating #298 step-by-step.

Builds on top of #324.

The grammar covers a subset of the IDL spec.

Linux CI:

[sloretz]

Seems fine to me.

I played around with it locally using

import os
path = os.path.dirname(__file__)
with open(os.path.join(path, 'grammar.lark')) as fin:
    grammar = fin.read()

from lark import Lark
parser = Lark(grammar, start='specification')

import code
code.interact(local=locals())

There are some odd things that I assume will be caught by something after the parser, like any literal can be assigned to any type.

# Parser accepts this
parser.parse('const double not_a_double = "Hello World";')

[dirk-thomas]

any literal can be assigned to any type.

Correct, that kind of check is out of scope for the grammar.

[dirk-thomas]

I disabled the union_dcl as well as the native_dcl which we both don't support atm: 95b264f