[FE]line number selection + typing /explain

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related to #461270 (comment 2013761770)

In scope of this issue, we need to Support the line number selection + typing /explain

• select multiple lines, (in mac shift+click to select multiple lines)
• in duo chat box, type /explain

frontend:

• translate the https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/spec/lib/gitlab/code_owners/validator_spec.rb#L29-38 into the actually code
• truncate if string is too long
• send the /explain_code request to backend

backend: Seems no backend code changes here