Pass argument to webview context menu action

[jonah-iden]

What it does

When executing a command via the context menu of a webview, vscode passes the context of the clicked html node to the command as an argument.
This PR aligns Theia's implementation of the webview context menu with vscode.

How to test

A small test extension can be found here.
The repo also contains a vsix file to download directly.

Go to the explorer tab, expand the Webview Test View webview. This contains two elements opening the context menu on each should return a different command because of their given context through the data-vscode-context property. Executing the command should now also include the stringified data from the vscode-data-context html-attribute

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

[martin-fleck-at]

LGTM! Thank you @jonah-iden!