Help me get started

I am working with a framework that has a custom file type called ".esp".    It is identical to a .jsp except the code between <% and %> is javascript instead of java.

I've tried playing around with assigning this .esp extension to various built in file types (xml, freemarker, velocity, javascript).   The one that comes the closest is velicity -- it shows everything outside of the <% %> tags (i.e. html) properly, without marking the code inside <% %> as bad.

But I'd like to take it a step further and have the code between the <%  %>  interpreted in the IDE as javascript.

How difficult would creating such a plugin be?  I'm an experience java developer but have never tried to write IDEA plugin before.   What resources can you point me to to get started?