Optimize result filtration.

[yavorsky]

Why?
For now, we have excess .filter(Boolean) while processing preset's result:
Firstly, there are extra checks for modules:

const modules = [
// false
  moduleType === "commonjs" && [require("babel-plugin-transform-es2015-modules-commonjs"), { loose }],
// false
  moduleType === "systemjs" && [require("babel-plugin-transform-es2015-modules-systemjs"), { loose }],
// false
  moduleType === "amd" && [require("babel-plugin-transform-es2015-modules-amd"), { loose }],
// true
  moduleType === "umd" && [require("babel-plugin-transform-es2015-modules-umd"), { loose }],
].filter(Boolean);
// Up to 4 checks

Then we iterate all modules array to get only single matched module:

modules.filter(Boolean)

By the end, we are processing new iteration by filtering preset's result:

plugins: [
  ...modules,
  ...plugins,
  useBuiltIns === true && [transformPolyfillRequirePlugin, { polyfills, regenerator }]
].filter(Boolean)

despite:

1. we've already checked modules array
2. plugins can't contain anything except array(it's result of allTransformations.map which returns array only and throws error if no plugin was found).
   So, we are iterating the whole array to check only last value.

How this PR optimizes aforesaid?

1. We already have MODULE_TRANSFORMATIONS to check and get module's plugin.
2. Replace array filtering with particular checks.
3. Fill up preset's result gradually.

[hzoo]

Kinda like just doing an if check in this case?

[hzoo]

👍