chore: Replace 'yarn' with 'npm'

[smockle]

This PR migrates PRC workflows and scripts from yarn to npm (specifically, to npm@6). Public-facing docs continue to give usage examples with both tools, since our customers use both.

Fixes https://github.com/github/primer/issues/179

Merge checklist

• Added/updated tests
• Added/updated documentation
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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Latest commit: 6bae722

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[smockle]

I’m very surprised (since the Tooltip test file hasn’t changed in months) that swapping to npm uncovered an accessibility-related issue that yarn elided (presumably by updating an axe-related dependency in the tree):

 FAIL  src/__tests__/Tooltip.tsx (5.329 s)
  ● Tooltip › should have no axe violations

    expect(received).toHaveNoViolations(expected)

    Expected the HTML found at $('span') to have no violations:

    <span aria-label="hi" class="Tooltip__TooltipBase-kp39nc-0 fhzzWU tooltipped-n"></span>

    Received:

    "Elements must only use allowed ARIA attributes (aria-allowed-attr)"

    Fix all of the following:
      ARIA attribute cannot be used, add a role attribute or use a different element: aria-label

    You can find more information on this issue here: 
    https://dequeuniversity.com/rules/axe/4.2/aria-allowed-attr?application=axeAPI

      18 |     const {container} = HTMLRender(<Tooltip text="hi" />)
      19 |     const results = await axe(container)
    > 20 |     expect(results).toHaveNoViolations()
         |                     ^
      21 |     cleanup()
      22 |   })
      23 |

      at Object.<anonymous> (src/__tests__/Tooltip.tsx:20:21)

Fixed in 6bae722

[smockle]

From @smockle in #1250 (comment):

• Tested in Safari

For (additional manual) verification (that is, beyond the automated verification from our test suite), I opened every PRC component page on https://primer-components-1dot5xcvk-primer.vercel.app/ (the latest Vercel deployment) and interacted with interactive elements demoed therein.

[dgreif]

Looks great, thanks for making this transition! Can't wait for npm7 this fall 🚀