update node version support in package.json

[sameerag]

Introducing "engines" in package.json for msal-node to update Node.js version support.

 "engines" : {
    "node" : ">=14.0.0"
  }

[coveralls]

Coverage increased (+12.05%) to 94.757% when pulling 6dc525f on add-node-version-support into 4ec58c4 on dev.

[jasonnutter]

Per our discussion last night, can we add a table to the readme (similar msal-angular) where we detailed which versions of Node are supported in which versions of MSAL Node? I think our policy should be when a major version of MSAL Node is released as stable, it should support all LTS and active versions (today that is 10-15, as done in this PR), drop support for any previously supported Node versions that have reached end of life, and should not support prerelease/preview/pending versions until they are stable, as denoted here: https://nodejs.org/en/about/releases/