Adds registry-url and always-auth parameters

[pzuraq]

These parameters match the ones passed to the setup-node GH action. The
implementation is mostly taken from there, with a few
updates/modifications (e.g. using fs/promises).

Fixes #47

[rwjblue]

Thanks for working on this!

Would you mind adding an "acceptance" test confirming that things work "end to end"?

I'm thinking something

action/.github/workflows/CI.yml

Lines 32 to 57 in 76620aa

	test-specific-volta:
	runs-on: "${{ matrix.os }}-latest"
	strategy:
	fail-fast: false
	matrix:
	volta-version: ["0.9.0","0.8.7","0.7.2","0.6.8"]
	os: [ubuntu, macOS, windows]
	exclude:
	# this action didn't support windows until at least Volta 0.7
	- os: windows
	volta-version: "0.6.8"
	steps:
	- uses: actions/checkout@v1
	- run: npm ci
	- run: npm run build
	- uses: ./
	with:
	volta-version: ${{ matrix.volta-version }}
	- run: tests/log-info.sh
	- run: tests/check-version.sh 'volta' ${{ matrix.volta-version }}
	- run: volta install node@10.17.0 yarn@1.19.0
	- run: tests/check-version.sh 'node' 'v10.17.0'
	- run: tests/check-version.sh 'yarn' '1.19.0'

Which might look like:

  test-specified-registry-url:
    runs-on: "${{ matrix.os }}-latest"

    strategy:
      fail-fast: false
      matrix:
        os: [ubuntu, macOS, windows]

    steps:
    - uses: actions/checkout@v1
    - run: npm ci
    - run: npm run build
    - uses: ./
      with:
        registry-url: "https://some.path.here.com/lol/"

    - run: tests/log-info.sh
    - run: tests/check-version.sh 'volta' ${{ matrix.volta-version }}
    - run: volta install node@10.17.0 yarn@1.19.0
    - run: tests/check-version.sh 'node' 'v10.17.0'
    - run: tests/check-version.sh 'yarn' '1.19.0'
    - run: tests/check-registry.sh 'npm' 'https://some.path.here.com/lol/'

tests/check-registry.sh can basically echo the file contents and then confirm via npm config get registry