Figure out Wasp's tsconfig story after Wasp 0.12.0

[sodic]

During the restructuring in Wasp 0.12.0, we had plans to optimize how Wasp uses different tsconfig.json files.

Wasp 0.12.0 ended up being a compromise between ease of implementation and DX. Now that 0.12.0 is out, we can focus on optimizing how Wasp uses and interprets tsconfig files.

Here's the original issues for context:

• Figure out Wasp's tsconfig story for the restructuring #1430

Other relevant context:

• Restructuring prototype RFC #1584 (comment)

TSconfig-related issues that are still relevant after 0.12.0:

• Ensure consistency between src/client/tsconfig.json and .wasp/out/react-app/src/tsconfig.json #1057 (no longer fully applicable, but this comment explains an approach to open questions).
• Support custom TypeScript configuration #835
• Ensure "user" tsconfig.json files exist and are correct #939
• Provide type checking in development #1718

[sodic]

Project references may help us have users' code compiled with their own tsconfig.json.:

• SDK: SDK includes a full copy of the user's code under node_modules/wasp/ext-src. We can also copy the user's tsconfig.json into node_modules/wasp/ext-src and make mode_modules/wasp/tsconfig.json point to it using a project reference.
  Possible problem: If parts of the config only work in the file's original location, we'll have a problem after copying it.
• Framework: Framework imports some components from ../../../src directly. We can use a project reference to include the user's tsconfig.json (which should already be applied to everything in src).

Reference: Effective Typescript, 2nd edition, item 78.