Fix web plugin initialization

[msujew]

What it does

Closes #12786

Instead of registering a custom static route handler to context, we simply move the web plugin initialization file to lib/frontend/context so that it can be handled by the general static frontend file handler. This makes the file fetching independent of whether the application backend is currently bundled.

How to test

1. Start the application and ensure that http://localhost:3000/context/plugin-vscode-init-fe.js serves the file correctly.
2. Install a web plugin and assert that it works as expected. See here for more info.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[tortmayr]

Looks good to me. The plugin-vscode-init-fe.js is now correctly servend and my test frontend plugin works as expected. 👍🏼