Adds tools/json_injector

[gaaclarke]

This adds a tool that will help reduce redundancy in //engine.code-workspace introduced in #55291. It does it by merging json files and applying templates.

I've introduced a tool that can merge json files instead of one that can just do templates since this will keep the source of truth in //engine.code-workspace. I don't think there is a way we can modify vscode to expand templates there.

Pre-launch Checklist

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• I listed at least one issue that this PR fixes in the description above.
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[gaaclarke]

closing in favor of #55322