Clarify imports

This commit:

- Adds `styles/` to all imports
- Adds `.less` to all imports
- Moves imports from `styles/*.less` to `index.less`

Reasoning: When importing other Less files in index.less it would be
possible to omit `styles` or `.less`, but to not cause any confusion
it's better to write out the full path.

Ref: atom/atom#13359

index.less

@@ -1,20 +1,20 @@
 
 // Atom Syntax Theme: One
 
-@import (reference) "styles/syntax-variables";
+@import "styles/syntax-variables.less";
 
-@import 'editor';
+@import "styles/editor.less";
 
-@import 'syntax/_base';
-@import 'syntax/c';
-@import 'syntax/cpp';
-@import 'syntax/cs';
-@import 'syntax/css';
-@import 'syntax/gfm';
-@import 'syntax/go';
-@import 'syntax/ini';
-@import 'syntax/java';
-@import 'syntax/javascript';
-@import 'syntax/json';
-@import 'syntax/ruby';
-@import 'syntax/python';
+@import "styles/syntax/_base.less";
+@import "styles/syntax/c.less";
+@import "styles/syntax/cpp.less";
+@import "styles/syntax/cs.less";
+@import "styles/syntax/css.less";
+@import "styles/syntax/gfm.less";
+@import "styles/syntax/go.less";
+@import "styles/syntax/ini.less";
+@import "styles/syntax/java.less";
+@import "styles/syntax/javascript.less";
+@import "styles/syntax/json.less";
+@import "styles/syntax/ruby.less";
+@import "styles/syntax/python.less";

styles/syntax-variables.less

@@ -1,4 +1,4 @@
-@import "colors";
+@import "colors.less";
 
 // Official Syntax Variables -----------------------------------