[dagit] Upgrade CodeMirror, fix error rendering
A minor upgrade to CodeMirror, but mostly this fixes a bug with how errors are displayed. A CodeMirror change introduced a new way of setting classes on lint marks, so we just need to reflect that in our own code.
Test Plan: View Playground in Dagit with errors, verify that backgrounds render correctly.
Reviewers: bengotow, sashank
Reviewed By: sashank
Differential Revision: https://dagster.phacility.com/D5136