Add tapioca/dsl entrypoint for custom DSL compilers

[paracycle]

Motivation

When writing a custom DSL compiler, the client code needs to require a minimal set of files from Tapioca, so that the DSL compiler can be loaded in isolation and tested inside the project. However, we don't have an entrypoint for that minimal require set and clients end up having to do require "tapioca/internal" to achieve this, which is not great.

Implementation

This PR adds a tapioca/dsl entrypoint, which DSL compiler classes can add as a require.

Tests

The test is super custom, since I wanted to test the experience of a client app declaring and loading a custom DSL compiler in isolation from the Tapioca CLI runner. When we load the Tapioca CLI dsl command and, that, in turn, loads custom compilers, everything works regardless of any Tapioca requires, since the CLI already bring everything with it.

The custom test defines a small executable inside the mock project that loads the custom compiler and tries to generate an RBI using it, just as how tests would do it. That allows us to test the loading and operation of a custom DSL compiler in isolation using only the minimal tapioca/dsl require.