Automatically share code font

This change moves code font to a shared selector so that it's automatically shared for any new platforms (such as Linux). This eliminates the need for each platform to add the following set of rules:

```
.platform-linux .CodeMirror-scroll {
    .code-font-linux();
}
.code-font-linux() {
    .code-font();
}
```

This is similar to change I made in @pritambaral branch: https://github.com/pritambaral/brackets/pull/1