Support VSCode web

[kanej]

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Our extension currently can't be installed on vscode for web. The web execution host doesn't allow file access in the way node does. We would need to restructure the plugin, to separate out features that can be used without node apis from those that can.

A starting move might be to turn on the webextension but only support syntax highlighting, then evaluate how much effort is required to get navigation, with that knowledge we can feed back to slang.