Use a subset of `strict: true` in the SDK

[Filip Sodić]

Building the SDK generates two kinds of files:

• JavaScript files (.js) used in the runtime.
• Declaration files (.d.ts) used for type support by the IDE.

Declaration files (d.ts) are subject to our user's tsconfig.json which is more strict than sdk/wasp/tsconfig.json used to build the SDK. This results in the SDK's declaration files having type errors when looked at from the user's perspective (e.g., during go-to-definition jumps).

We've avoided using strict: true in the tsconfig.json we use for building the SDK due to legacy reasons (old JS or improperly typed code that still works).

This is not a big problem, but definitely isn't ideal:

• Our SDK is deemed "wrong" by our own system, which is a little unprofessional.
• These type errors truly are type errors (they're not false-positives). Not compiling the SDK in strict mode prevents us from catching them. In other words, a small part of legacy code disqualifies the entire SDK from reaping the full benefits of TypeScript.

Since the strict: true compiler flag is a shorthand for multiple more specific flags, we should activate all the "subflags" that don't require a major refactor of the SDK. After that's done, we can tackle #1938 and change whatever we need to go full strict mode (this will probably take some time).