Implement VS Code extensions namespace

[thegecko]

Signed-off-by: thegecko rob.moran@arm.com

What it does

This PR implements the extensions namespace in the VS Code API which is currently marked as not implemented in the comparison coverage report.

The functionality was already implemented, but exposed under the plugins export.

How to test

You can test the extensions onDidChange event by hooking into it in a VS Code extension.
You can test the getExtension function by calling it from a VS Code extension. You can then activate this extension and access the functionality on it using this pattern:

https://github.com/Marus/cortex-debug-db-stm32f4/blob/master/src/extension.ts#L15

Note: access to plugins is currently only possible within the same extensionHost (this matches VS Code functionality)

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

[msujew]

Looks good to me 👍 I can confirm that the API comparison report marks the API as fully supported.

[tsmaeder]

@thegecko the vscode namespace was implementing the extensions namespace in terms of the plugins namespace in plugin-vscode-init.ts. Shouldn't we remove that "hack" now?

[thegecko]

Good spot, this should probably be removed.