node-version as fallback of node-version-file

Description

Currently the behavior is that node-version is preferred rather than node-version-file if both are provided.

https://github.com/actions/setup-node/blob/d86ebcd40b3cb50b156bfa44dd277faf38282d12/src/main.ts#L85-L95

I would like to propose the opposite since node-version is actually hardcoded in the action parameters, but the file is not guaranteed to be there.

https://github.com/actions/setup-node/blob/d86ebcd40b3cb50b156bfa44dd277faf38282d12/src/util.ts#L8-L12

Currently, scenarios resolved as following:

with:
  node-version: 8 # nothing to see here

with:
  node-version-file: fileExists.json # all good

with:
  node-version-file: fileDoesNotExists.json # throws error and crashes the pipeline

with:
  node-version: 8
  node-version-file: fileExists.json # ignored, picked version is 8

with:
  node-version: 8
  node-version-file: fileDoesNotExists.json # ignored, picked version is 8

What I'm requesting for is a change in the 2 lasts scenarios:

with:
  node-version: 8
  node-version-file: fileExists.json # has priority, so it's picked

with:
  node-version: 8
  node-version-file: fileDoesNotExists.json # does not throw and picks version 8 instead

Justification

This is just a nicer API, i think. it does not remove the case of conscious crash of the pipeline if no node-version is given, but it does nicely propose a fail-safe for people interested.

Note

We could also have another parameter such as node-version-fallback-if-file-not-found but it'd just increase the complexity of the api.

Are you willing to submit a PR?

Yes, i'm ok with that.

Start in src/main.ts at lines 85-95 to trace how node-version and node-version-file are selected, then read src/util.ts lines 8-12 for the file lookup behavior. Done means an existing file takes priority, while a missing file falls back to node-version when both inputs are provided without crashing the pipeline.