core-js-builder: Allow webpack 5 and modernize code

[susnux]

Current node LTS 14.x is already about to be EOL, so it should be safe to update the minimum node version to at least 12.x
This allows to modernize the code and get rid of a dependency (mkdirp).

Also allow using webpack 5 to fix issues reported by npm audit when using the core-js-builder:

6 high severity vulnerabilities

# npm audit report

glob-parent  <5.1.2
Severity: high
glob-parent before 5.1.2 vulnerable to Regular Expression Denial of Service in enclosure regex - https://github.com/advisories/GHSA-ww39-953v-wcq6
fix available via `npm audit fix --force`
Will install core-js-builder@3.6.5, which is a breaking change
node_modules/watchpack-chokidar2/node_modules/glob-parent
  chokidar  1.0.0-rc1 - 2.1.8
  Depends on vulnerable versions of glob-parent
  node_modules/watchpack-chokidar2/node_modules/chokidar
    watchpack-chokidar2  *
    Depends on vulnerable versions of chokidar
    node_modules/watchpack-chokidar2
      watchpack  1.7.2 - 1.7.5
      Depends on vulnerable versions of watchpack-chokidar2
      node_modules/watchpack
        webpack  4.44.0 - 4.46.0
        Depends on vulnerable versions of watchpack
        node_modules/webpack
          core-js-builder  >=3.7.0
          Depends on vulnerable versions of webpack

With this PR: found 0 vulnerabilities

[zloirock]

Sorry, I can't accept it - core-js-builder@3 should support NodeJS 8 even after EOL, otherwise, it's a breaking change. Anyway, I wanna start active work on core-js@4 with tooling soon where such dependencies will be updated / replaced / removed.