Load and run workers with installed runtimes

[Angelmmiguel]

Now that developers can install new runtimes (#68), wws should pick the installed ones and run workers based on it. For that, it will identify the new file extensions to look for files and then, prepare the worker and configure the WASI context.

If a language runtime is present in the .wws.toml config (see #67) and it's not installed, wws will show a warning message indicating they are not installed and some workers won't run.

This will require a new src/runtimes/runtime/external.rs struct that properly configures the installed language runtime and the WASI context. All this must come from metadata as developers may create their own set of runtimes.