Treat emmet snippets as textmate

./features/implementationProvider or ./features/ImplementationProvider ?

Queuing a large number of WinJS promises that aren't actually async can lead to a stack overflow. Also this is faster.

Fixes #24688

Fixes #24630
Related #19665

Fixes #24898

Fixes #24808

**bug**
Markdown preview updates when you zoom. The root cause is that previews are updated whenever the config is changed.

**Fix**
Extract preview config to its own well defined object. Only update the preview when the keys we care about in the config change

This reverts commit 48722f67.

This reverts commit f3653925.