Support jsdoc `@implements` tag

[sandersn]

Add support for the jsdoc @implements tag. The code will not look much like the Strada support; it just needs to add a case in reparser.go for JSDocImplementsTag. There, it needs to create a synthetic heritage clause that contains the entity name from the tag. The tag was a late addition in Strada, so there shouldn't be any strange checking rules; that should be nearly all the required changes.