Add support for SnippetString in TextEdit and WorkspaceEdit

[jrieken]

The TextEdit and WorkspaceEdit APIs are used to apply refactorings. Those will benefit from being able to place the cursor and add placeholders, like add a placeholder for the suggested name of an extracted vairable

[jrieken]

For April I have enabled this (for editor dnd) and for May we can make this more prominent. Questions

• how to handle N snippet edits
• where to allow snippets edits
• how do snippet edits fit into workspace edits