Eclipse doesn't use JEditorPane, JTextPane, or anything else like it; it uses its own window toolkit called "SWT", which has a native-method component.
Eclipse editors are enormously complex; the functionality is spread across hundreds of packages. The base GUI widget is called "StyledText", but highlighting, line numbering, etc, are all added by other code in other places.
Here is an article on StyledText itself, but the place you really want to start is the "Platform Plugin Developer's Guide" in the Eclipse Help. There are a number of excellent books on programming Plugins for Eclipse; have a look in the JavaRanch Bunkhouse to find them.