fix(webpack-plugin): create output folder with mkdirp

[jackhsu978]

Summary

If neither outputFolder nor the directory of outputFolder exist, then outputFolder would not be available for write. For example, if outputFolder is /a/b/c, but /a/b does not exist, then the plugin cannot create directory /a/b/c and cannot write any files to it.

Test plan

I tested the razzle example in the repo with the following steps:

1. in examples/razzle/razzle.config.js, change const filename = path.resolve(__dirname, 'build') to const filename = path.resolve(__dirname, 'build/a/b/c')
2. run yarn start and observe that it results in an error Error: ENOENT: no such file or directory, mkdir ...
3. use the patched plugin and try yarn start again, observe that there is no error this time, the folder build/a/b/c is created, and loadable-stats.json is written to build/a/b/c

[gregberge]

It makes sense, thanks!