Update gatsby-plugin-netlify to allow caching JS files via HTTP headers

[tanvibhakta]

This PR closes #9151

Adds a line to apply recommended caching behavior on all /src/ files in a gatsby project served on netlify, increasing cached content and therefore performance. This also makes the plugin consistent with the documentation.

[pieh]

this is bit confusing - we will not have src directory in built site, can You elaborate a bit on how this works? I thought it should use paths inside public directory that get served by netlify hosting

[tanvibhakta]

You are right, and I completely overlooked that. Will fix and re-push!

[tanvibhakta]

Unsure why the 'no-useless-escape' is being activated, without the escape charecter non-js files with 'js' in the name are also cached.

[pieh]

There is edge case that will be problematic with just caching all .js files - there might be scenario where there are .js files that are not generated by gatsby webpack in output directory and we don't want to cache them permanently.

We will need to read webpack manifest, extract .js (and I think .css files too) and we will want to cache those.

Unsure why the 'no-useless-escape' is being activated, without the escape charecter non-js files with 'js' in the name are also cached.

eslint isn't aware that this will be used as Regular Expressions, so we would need to opt out of that check in those lines. But we will also probably will need to drop that regex and target specific files because of my top part of my comment.

[DaleLJefferson]

Why are .js files not in the static folder?

[chadfawcett]

@pieh Would the list of assets found at assetsByChunkName in the public/webpack.stats.json file cover all the js and css assets that should be cached indefinitely?

[wardpeet]

Would love to get this one in! @tanvibhakta if you feel like processing this feedback that would be great. It's probably better to open up a new branch from master and try @pieh's feedback.