monaco_editor: support editing YAML

we pass the schema already, but for now this only supports basic
syntax highlighting
8 jobs for feat/open-api in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:17:58

passed ERP5.PerformanceTest-Master

failed ERP5.UnitTest-Master

02:09:42

failed ERP5.UnitTest-Zope2

04:02:35

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python3

00:15:59

passed Wendelin.UnitTest-Master

00:26:01

passed Wendelin.UnitTest-Master

03:35:15