Add custom HTML views to VSCode module

[daviwil]

This change adds support for custom HTML views in the VSCode module
which forms the basis for UI extensions in the PowerShell extension in
Visual Studio Code. This change includes both .NET PowerShell
cmdlet APIs for interacting with custom HTML UI in the editor.

[daviwil]

I'm not fully done here yet, I think I need to rethink the HtmlContentView/HtmlOutputView split because there's a lot of unnecessary API duplication there. Any thoughts?

I'll send the vscode-powershell PR after I've firmed this one up.

[daviwil]

A few firsts in this change:

• This is the first time a new feature component has been added from an extension module
• This is the first time that new message types have been added from an extension module
• This is also the first attempt at making a cmdlet wrapper on top of the .NET APIs instead of exposing the .NET APIs directly for PowerShell scripting

Would love some feedback on any of these since I'm still trying to formalize the pattern.

[dfinke]

I like the sound of a message type being added from an extension module

[SeeminglyScience]

Great job with the wrappers, these will be a lot easier to discover from PowerShell.

[daviwil]

What are your thoughts on using a DefaultCommandPrefix of VSCode? This means that all cmdlets like New-HtmlContentView will turn into New-VSCodeHtmlContentView when imported. Even though prefixing is considered somewhat of a best practice, I feel like it makes the cmdlet names a lot clunkier and harder to type. Thoughts?

[dfinke]

Yeah, clunkier. I do like the option of being able to do gcm *-vsc* and seeing all the cmdlets I can use.

[daviwil]

The ISE only exposes 3 cmdlets but they're all prefixed "Ise", so maybe prefixing is generally the right thing to do. I'll hardcode the prefixes for now instead of using DefaultCommandPrefix so that I have the opportunity to change the name later and use aliases for the old prefixed versions. Thanks Doug!