Finalize the PG CodeMirror Editor for the 2.20 release.

A non beta version 0.0.1 of the `@openwebwork/pg-codemirror-editor` has
now been published.  So make that the version for the 2.20 release.  If
there are any other changes the version can still be bumped if needed,
but I doubt that there will be at this point.

The `@openwebwork/codemirror-lang-pg` package that the
`@openwebwork/pg-codemirror-editor` depends on has a non beta version
published (version 0.0.1).

The `codemirror-lang-perl` and `codemirror-lang-mt` packages are now
also published in non beta status. Those are under my name still at this
point since they are not specific to the openwebwork project.

Author: drgrice1