--experimental modules flags should have warnings

[guybedford]

Currently the following experimental modules flags don't have warnings:

• --experimental-json-modules
• --experimental-wasm-modules
• --experimental-resolve-self
• --experimental-conditional-exports
• --es-module-specifier-resolution=node

Ideally when using any of these flags we should provide a warning message that not only is the base experimental modules implementation unstable, but these flags are not even clear if they will be provided as final features in the implementation and any of them might be removed still.

[pd4d10]

If multiple experimental flags specified, should these warnings emitted separately for every flag, or merged into one?

[guybedford]

I would prefer one for each. Warning consolidation is a separate consideration for Node.js more generally I think.

[guybedford]

For at least --experimental-json-modules, --experimental-wasm-modules it should be possible to move the warnings to their first use.

For --experimental-resolve-self and --experimental-conditional-exports the first use would be within C++ code and applies both to the CJS and ESM resolver implementations so may be a bit trickier, but it is in theory possible.