feat[venom]: add venom parser

[Philogy]

What I did

I added a "frontend" for venom, allowing you to compile independent venom files.

How I did it

I defined a new grammar (a bit simpler than what the readme initially suggested) & parser using the lark parser generator library.

How to verify it

Run the venom CLI against the example in the readme by putting it in a example.venom file and running venom example.venom

Commit message

this commit adds a frontend (parser) for venom. it uses the lark
library to define a grammar for venom, and constructs an `IRContext`
which can be used to emit bytecode.

the entry point to the venom compiler is `vyper/cli/venom_main.py`.

possible improvements in the future include:
- make data section optional in the grammar
- make the entry block optional in the grammar (a la llvm)
- add asm and opcodes output formats

Description for the changelog

Added independent CLI for compiling venom

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Package	New capabilities	Transitives	Size	Publisher
pypi/lark@1.2.2	eval, filesystem, unsafe	0	704 kB	erez

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[harkal]

🚀

[Philogy]

@charles-cooper btw how I would I toggle optimizations on-off for faster venom compilation?

[charles-cooper]

very slick -- much simpler than i was expecting! left a few comments

[charles-cooper]

i think it would be good to have a couple test cases! e.g. in tests/functional/venom/parser/

[Philogy]

@charles-cooper should be ready to merge now

[charles-cooper]

lgtm. a couple things for future scope:

• make data section optional in the grammar
• make entry block optional in the grammar (similarly to llvm)
• more output formats (-f asm, opcodes, bytecode)