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://github.com/sprintr/brackets-svg-code-hints) 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.

I didn't think `_setLanguageForMode()` was necessary since the SVG Language is defined in **languages.json**

I ran those commands from console and got what I expected: Language is SVG and mode is XML.

**TokenUtils.getModeAt(EditorManager.getCurrentFullEditor()._codeMirror, {line: 5, char: 16})**
```
  Object {mode: Object, name: "xml"}
    mode: Object
      blockCommentEnd: "-->"
      blockCommentStart: "<!--"
      configuration: "xml"
      electricInput: /<\/[\s\w:]+>$/
      helperType: "xml"
      indent: function (state, textAfter, fullLine) {
      name: "xml"
      startState: function () {
      token: function (stream, state) {
    name: "xml"
```

**EditorManager.getCurrentFullEditor().getLanguageForSelection()**
```
  Language {_fileExtensions: Array[1], _fileNames: Array[0], _modeToLanguageMap: Object, _lineCommentSyntax: Array[0], _id: "svg"…}
    _blockCommentSyntax: Object
    _fileExtensions: Array[1]
    _fileNames: Array[0]
    _id: "svg"
    _isBinary: false
    _lineCommentSyntax: Array[0]
    _mode: "image/svg+xml"
    _modeToLanguageMap: Object
  _name: "SVG"
```

I've been testing this using the [SVG Code Hints extension](https://github.com/sprintr/brackets-svg-code-hints) which has it's own **XMLUtils.js** module.