Separate the Reason parser from the JS compiler bundle

[ryyppy]

Due to the way the Reason v3.6 was designed, the resulting JS code of just the parser alone is twice the size of the whole ReScript compiler + syntax. Since users like @bobzhang are oftentimes in slow latency networks, shaping off more than half of the bundle size would heavily decrease load times.

Here is a perf benchmark made by Bob:

Since Reason is pretty much stuck on v3.6, it would probably be a good idea to compile the Reason bundle separately, and plug the compiler + syntax back together on the frontend after lazily loading the Reason parser.

Steps to make this work:

• Find a way to compile the Reason parser separately and provide one single bundle (probably in a separate repo for now)
• Publish the Reason v3.6 bundle on our CDN
• Hook up the parser and the compiler so they can interact with each other on the playground (verify if it works)
• Change the playground logic to only load the Reason bundler when the user presses the RE button

Steps in the ReScript JSOO bundle:

• Remove refmt completely
• Adapt the frontend facing APIs

This will greatly reduce the complexity in the ReScript compiler repo, and make the playground loading experience way more performant.

[fhammerschmidt]

This is basically solved. refmt is not part of the jsoo bundle since a long time and the Reason parser as part of the compiler only gets loaded when legacy versions like ReScript 9 or earlier are selected.