[Request] Allow Snippets in WorkspaceEdit's

[NikolasKomonen]

Somewhat related to this issue: #715

It's been recommended to use workspace/applyEdit to enable automatic completion/insertion of text.

The issue is workspace edits do not allow Snippets, which is something that Completion Items use (InsertTextFormat) to indicate plain text or snippets.

[rcjsuen]

See #592 and #724.

[dbaeumer]

I mark this as a dupe of #724 since it already has a lengthly discussion about the issues around this and how to address them. They could be mimicked for workspace edits.