[syntax-highlighting] Change / Add syntax highlighting for a language in Sublime 2/3

Use the PackageResourceViewer plugin installed via Package Control (as mentioned by MattDMo). This allows you to override the compressed resources by simply opening it in Sublime Text and saving the file. It automatically saves only the edited resources to %APPDATA%/Roaming/Sublime Text 3/Packages/ or ~/.config/sublime-text-3/Packages/.

Specific to the op, once the plugin is installed, execute the PackageResourceViewer: Open Resource command. Then select JavaScript followed by JavaScript.tmLanguage. This will open an xml file in the editor. You can edit any of the language definitions and save the file. This will write an override copy of the JavaScript.tmLanguage file in the user directory.

The same method can be used to edit the language definition of any language in the system.