Type check Emscripten JS overrides and allow changes without php-wasm rebuilds

[brandonpayton]

Motivation for the change, related issues

Our system call overrides are not type checked, and any change to them requires rebuilding php-wasm JS files.

If we can separate our customization into TS modules and inject them into the php-wasm JS, we can benefit from type checking and reduce the number of slow php-wasm rebuilds we perform.

Implementation details

TBD

Testing Instructions (or ideally a Blueprint)

• CI

[brandonpayton]

At first, I thought we could make our php-wasm JS customizations (i.e., our fcntl() override) more testable by separating them into TS modules that can be examined independently and injected into php-wasm JS at run time.

But as I explore these changes, it looks like the most economical way to test our customizations is in their usual context with further integration tests.

That said, separating into TS modules would:

• Make critical php-wasm overrides subject to type checking
• Reduce php-wasm rebuilds. The main reason a rebuild would be required when our customizations change is if their bindings require additions (for example, relaying a new constant from Emscripten cDefs).

So I plan to continue working on this PR, but for the moment, I will pivot to:

• Add simple integration tests for file locking between multiple php-wasm instances.