Parse simple programs (integer, string, float, or rational) with Prism

[egiurleo]

This PR is best read commit-by-commit.

Motivation

1. Add the --parser=prism flag to the Sorbet CLI options, allowing the user to choose to parse with Prism
2. Depending on the value of this flag, the pipeline diverges at the parsing stage and parses with prism instead of the Sorbet parser
3. Prism parses the source, then walks through the resulting AST in order to convert it into the Sorbet AST format
4. So far, it can handle Program, Statement, Integer, Float, Rational, and String nodes 😁

Manual tests

You should be able to compile Sorbet:

./bazel build //main:sorbet --config=dbg --define RUBY_PATH="/path/to/your/ruby"

Then typecheck any Ruby program that is ONLY an integer, string, rational, or float, e.g.:

bazel-bin/main/sorbet -e "42" --parser=prism 
bazel-bin/main/sorbet -e "1.0r" --parser=prism
bazel-bin/main/sorbet -e "'hello world'" --parser=prism 
bazel-bin/main/sorbet -e "4.2" --parser=prism 

Automated tests

Still working on this part!

[kddnewton]

A great start, nice work!

I've added a bunch of comments. They can be addressed now or later, since this is WIP I'm not overly worried about them at this point. Most of them were just to convey information as opposed to request changes.

[amomchilov]

Sick!!!

[amomchilov]

Could you please switch this to a switch (pun intended), and add a default case that errors-out?

If we hit that, it indicates that we changed main/options/options.cc, but forgot to handle the new value here.

[egiurleo]

So I attempted to implement this because it makes sense, but I realized that Sorbet is implemented in a way that would actually make it impossible to reach the default part of the switch statement. I can program the options.cc file to recognize if an invalid parser option is passed, then just use the default option (sorbet) instead, making any further processing unnecessary.

[egiurleo]

@Morriar @amomchilov @KaanOzkan how would you feel about merging this? I'm setting up an automated testing pipeline but the changes are going to be somewhat significant because testing with Bazel is complicated 😓 I don't want to have to add a lot more changes to this PR if I don't have to.