Refactor extension

[dselman]

I think this should be renamed to accordproject-vscode-extension and have the following features:

• edit/validate tem files
• edit validate cto files
• edit/validate ergo files
• integrate the Ergo REPL?
• generate UML for a template
• generate HTML description of a template
• parse a file using a template
• execute a template against request and state

We should rework the validation logic to be aware of the template that is being worked on so that ModelManager that is created is specific to a single template.

This will mean that Accord Project users won't have to install the Composer extension, which does a lot more than edit models.

[jeromesimeon]

About Ergo REPL: what would be the API/Hooks inside VSCode, if any available?

[mttrbrts]

As per the @dselman's comment on #13, this refactor should allow us to

handle both CTO and Ergo files and [be] smarter about incremental validation and downloading external dependencies.