chore(gatsby): migrate bootstrap load-themes to typescript

[nategiraudeau]

Description

Migrates packages/gatsby/src/bootstrap/load-themes to TypeScript.

Related Issues

Related to #21995

[imjoshin]

Hey! Thanks so much for opening this pull request!

Sadly this PR got stale and didn't have any activity for some time. We're trying to do better with PR reviews! To get a better overview of all actionable PRs we're going through all open PRs and triage them. Since we won't be able to do everything and adding new features always means added maintenance burden, we have to be more picky about what's beneficial for the average user and the project itself longterm.

We think this is a great PR and would love to see it land in Gatsby. We're closing this PR for now but if you're able to rebase onto the latest master branch and try it out with the latest next versions, we'd be happy to review the PR when its re-opened.

We absolutely want to have you as a contributor and are sorry for any inconveniences we caused with replying too late to this PR.

Thanks for submitting to Gatsby! 💜