Enable SVG Code Hints

This pull request is Step 1 of this plan for improved SVG support:

1. Split SVG into own language (i.e. This PR)
2. Update [Brackets SVG Code Hints extension](https://s3.amazonaws.com/extend.brackets/io.brackets.svg-code-hints/io.brackets.svg-code-hints-1.0.0.zip) because it's no longer required create an SVG language. Also update documentation because it's no longer necessary to manually switch to SVG language
3. **Bonus**: Merge SVG Code Hints into Brackets Core (after code cleanup, adding some unit tests, and code review).

There are some other SVG extensions in Brackets Registry that may also benefit from this change.

Is the `helperType` needed?  I think that was just part of Marijn's suggestion for CM-based hints, but it shouldn't be needed for ours.  If we don't need it, I think this line can be simplified to just:

```
CodeMirror.defineMIME("image/svg+xml", "xml");
```