To find extensions, select "Extension Library" from the "Extensions" menu.
Not entirely sure what you mean by "run a file," but under "Project Settings" ("Project" menu) you can set up scripts for building, running, and cleaning a project, and after you do so buttons to run those scripts appear in the toolbar.
In TextMate, you can say, run a Perl/Python/PHP script within the editor, and have the output printed in a new window. r just get a syntax check done on the file you're working on. Convenient.
TextMate is in comparison to Nova a little more "batteries included" in that sense - those extensions are already written and bundled within the app.
Without those tools in place, it's harder to jump ship to Nova.
Ah. Well, that's still something you can do with Tasks in a manner; just write a line of shell script to invoke the interpreter with the path to the desired file, and select "On Finish" from the "Open Report" menu that's near the bottom when you edit a task.
Not entirely sure what you mean by "run a file," but under "Project Settings" ("Project" menu) you can set up scripts for building, running, and cleaning a project, and after you do so buttons to run those scripts appear in the toolbar.