Fix issue 391

[cfehse]

Fixes #391.

Introduces the following change(s):

• Updated package.json to use @types/vscode package in the same version a specified in the engines definition.
• Removed type/compiler error which was caused by this update in docmirror/index.ts
• Change URI generation in repl-windows.ts to use the webview.asWebviewUri() method.

I have:

• Read How to Contribute.
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• Made sure I am changed the default PR base branch, so that it is not master. (Sorry for the nagging.)
• Referenced the issue I am fixing/addressing in a commit message for the pull request.
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• Created the issue I am fixing/addressing, if it was not present.
• Updated the [Unreleased] entry in CHANGELOG.md, linking the issue(s) that the PR is addressing.

Ping: @PEZ, @kstehn, @cfehse