feat(gatsby-plugin-netlify): Add caching headers for immutable assets

[cameron-martin]

Description

This PR adds long-term caching headers to the _headers file for any js files that have a hash in their filename. These are only added if the existing mergeCachingHeaders setting is enabled.

Related Issues

Fixes #13902.

[wardpeet]

This looks awesome! This is great for our netlify users, it will increase speed for many regular visitors.

I'm checking to make sure it all works 👌

[wardpeet]

Looks good. This will enable faster gatsby sites on netlify 💪

[gatsbot]

Holy buckets, @cameron-martin — we just merged your PR to Gatsby! 💪💜

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[wardpeet]

published in [email protected] 🎉