[plugin] basic support of snippets

[akosyakov]

fix #3972
fix #3976 in order to load bat built in extension for testing

In order to try: check out, build, run browser example, create bat file and use content assist.

The changes to the browser-example and bat extension will be reverted before merging.

[akosyakov]

@benoitf @evidolob @svenefftinge it is ready for the review. The last commit will be removed after approval.

[svenefftinge]

Would this supercede https://github.com/theia-ide/theia/blob/master/packages/monaco/src/browser/textmate/textmate-snippet-completion-provider.ts as well?

[akosyakov]

@svenefftinge yes, TextmateSnippetCompletionProvider can be deprecated

[akosyakov]

@svenefftinge i've deprecated it
@benoitf i've changed loading of snippets, now they are loaded async

I've tested without and with plugin system. TS snippets from the TS extension are available as before and bat snippets are loaded async. If no one objects i merge it when travis build is green.

[benoitf]

no objection on my side

[benoitf]

will it be committed ?

[akosyakov]

I found myself doing it over and over, and thought that we can specify relative plugins location for testing purposes.

Actually, i think maybe we should add a new example that includes only plugin-ext-vscode and its dependency, and exclude all irrelevant Theia extensions.

// cc @svenefftinge