Remove or move `src/vs/loader.js`

[bpasero]

Given https://github.com/microsoft/vscode-dev/issues/1245#issuecomment-3675865128, I am moving ahead to remove the ESM-AMD bridge support for web in #285230. This means web no longer has a dependency on src/vs/loader.js and thus can be removed or moved.

However, it looks like there are still certain uses (searching for loader.js in vscode):

• the editor build (?)
• the playground
• some tests

So maybe we still need to preserve it but then I would suggest to move it out of src/vs to make it clear its only used for dev purposes.

//cc @alexdima