Add support to VS Code style snippets in code completion

[tsmaeder]

What does this PR do?

Adds support for VS Code-style snippets in code completion.
See https://github.com/Microsoft/vscode/blob/0ebd01213a65231f0af8187acaf264243629e4dc/src/vs/editor/contrib/snippet/browser/snippet.md

What issues does this PR fix or reference?

#5365

Changelog

Add support for snippets in code completion.

Release Notes

This version introduces new capabilities in the support of Language Server Protocol in Eclipse Che, we are now supporting the VS Code-style snippets in code completion.
Code snippets are small templates of reusable code that can make easier and faster to insert code in a file. Templates are particularly efficient for loop blocks, if-else blocks but not only depending the language you are using. When using the autocompletion ctrl+space you'll have the list of snippets suggested.

The snippets syntax support:

• Tabstops: to move the editor's cursor inside a snippet.
• Variables: to insert value of a variable.
• Placeholders: to easily insert placeholder text which will be selected to be changed (placeholders are tabstops with values).
• Choices: to replace a placeholder by a value from a given enumeration.

[Insert Animated gif for "choices"]

You can refer to the following documentation to learn more. We are currently supporting the VS-Code style syntax description.

Docs PR

eclipse-che/che-docs#292

[codenvy-ci]

Can one of the admins verify this patch?

[codenvy-ci]

Can one of the admins verify this patch?

[evidolob]

ci-build

[codenvy-ci]

Build success. https://ci.codenvycorp.com/job/che-pullrequests-build/3723/

[slemeur]

@tsmaeder : I've edited the RN. Would it be possible to attach an animated gif showing the code snippet with a "choice". I think it would nicely illustrate the new capability.

Also, are we currently already integrated LS which are using snippets?

[tsmaeder]

Here's the demo:
The only one I'm aware of is the json LS,