Add syntax-variables.less

From Atom's [template guide](https://github.com/atom/template-syntax#notes-and-best-practices):

> Define proper colors in your syntax-variables.less file. Other packages can use these variables. All the variables defined in this package's syntax-variables.less file must be defined — you cannot pick and choose.

This is useful in [markdown preview](https://github.com/atom/markdown-preview/commit/5b4af3a98bb92979a9ab6af3a5f93e8b237f343a), for instance.

### Colors I wasn't sure which one would be correct:

- Wasn't sure about `@syntax-selection-flash-color`. I used `@search-active-bg`.
- Wasn't sure about `@syntax-result-marker-color` and `@syntax-result-marker-color-selected`. I used `@code-font-color` and `white`.
- Other colors should be fine.