Add importable scripts

[cvanem]

Summary

RFC #116
Documentation: yarnpkg/website#951

Allow scripts to be imported from a file as an alternative to the standard object hash in the packages.json scripts field. Also define the basic structure of the imported file.

Test plan

The code was tested with this example input file:

module.exports = {
  delimiter: '.',
  scripts: {
    argos: 'argos upload test/regressions/screenshots/chrome --token $ARGOS_TOKEN',
    docs: {
      api: {
        default: {
          script: 'yarn docs.api.clean && yarn docs.api.mui && yarn docs.api.lab',
          description: 'Generates the api documentation',
        },
        clean: {
          script: 'rimraf ./pages/api',
          description: 'Removes all previously generated documenation',
          watch: {
            script: 'yarn docs.api.clean --watch',
          },
        },
      },
      build: {
        default: {
          script: 'yarn docs.build.clean && yarn docs.build.compile',
          description: 'Builds the documents.',
        },
        clean: {
          default: 'rimraf .next',
          watch: 'rimraf .next --watch',
        },
        compile: 'cross-env NODE_ENV=production BABEL_ENV=docs-production next build',
      },
      typescript: {
        default: 'node docs/scripts/formattedTSDemos --watch',
        check: 'tslint -p docs/tsconfig.json',
        formatted: 'node docs/scripts/formattedTSDemos',
      },
    },
    typescript: 'lerna run typescript && yarn docs.typescript.check',
  },
};

Which generates these scripts when executing yarn run:

[buildsize]

File name	Previous Size	New Size	Change
yarn-[version].noarch.rpm	1.11 MB	1.11 MB	812 bytes (0%)
yarn-[version].js	4.48 MB	4.48 MB	3.24 KB (0%)
yarn-legacy-[version].js	4.67 MB	4.67 MB	3.69 KB (0%)
yarn-v[version].tar.gz	1.12 MB	1.12 MB	656 bytes (0%)
yarn_[version]all.deb	816.96 KB	817.3 KB	358 bytes (0%)