@size-limit/webpack: add pnp support

[pustovalov]

before:

[~/site-new]$ yarn size                                                                                                                                                      *[yarn2][ruby-2.6.1]
 WARN  You can remove size-limit dependency
       All plugins and presets already contain it as own dependency
       yarn remove size-limit

 ERROR  Error:
ERROR in multi ./build/app.2dc4b36696c9586571f9.css
Module not found: Error: Can't resolve 'style-loader' in '/Users/pavel/site-new'
 @ multi ./build/app.2dc4b36696c9586571f9.css index[0]
    at /Users/pavel/site-new/.yarn/cache/@size-limit-webpack-npm-3.0.0-6b1f9f999d-1.zip/node_modules/@size-limit/webpack/run-webpack.js:8:23
    at finalCallback (/Users/pavel/site-new/.yarn/cache/webpack-npm-4.41.5-ec6f53e9d3-1.zip/node_modules/webpack/lib/Compiler.js:257:39)
    at /Users/pavel/site-new/.yarn/cache/webpack-npm-4.41.5-ec6f53e9d3-1.zip/node_modules/webpack/lib/Compiler.js:273:13
    at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/pavel/site-new/.yarn/cache/tapable-npm-1.1.3-f1c2843426-1.zip/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:4:1)

after:

[~/site-new]$ yarn size                                                                                                                                                      *[yarn2][ruby-2.6.1]
 WARN  You can remove size-limit dependency
       All plugins and presets already contain it as own dependency
       yarn remove size-limit


  build/js/app.9901f952.js
  Size limit:   4 KB
  Size:         3.4 KB with all dependencies, minified and gzipped
  Loading time: 68 ms  on slow 3G

  build/js/vendors.419c86c7.js
  Size limit:   15 KB
  Size:         13.85 KB with all dependencies, minified and gzipped
  Loading time: 277 ms   on slow 3G

  build/app.2dc4b36696c9586571f9.css
  Size limit:   10 KB
  Size:         9.55 KB with all dependencies, minified and gzipped
  Loading time: 191 ms  on slow 3G

[ai]

Does it mean that we got a Yarn 2 support?

[pustovalov]

Partiality, this issue #154 still relevant

[ai]

I would like to close #154 first before releasing this PR