This adds additional "Editor" service method definitions as used by the new DTD sidebar. I moved the Editor service definitions into a new file (because they're now quite large and dtd_common_services seems better as an intro and list of the common specified services).
I also split the existing info about `navigateToCode` into a more terse definition (in the new file) and a general example. I also updated the error example as I believe it was incorrectly wrapped in `result`.
Change-Id: I6136eb550cce0dab86d535db02f9f06ee42892e0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/378141
Reviewed-by: Kenzie Davisson <kenzieschmoll@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>
Reviewed-by: Helin Shiah <helinx@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>