.github: Complete migration from CODE_OWNERS.txt

[AnthonyLatsis]

• @akyrtzi Do you want me to assign you to IDE, Index, SourceKit, and swift-ide-test files per the old CODEOWNERS.txt?
• @belkadan Do you by chance know who took charge of serialization & frontend files? Do you want to receive notifications about PRs that modify these files?
• @DougGregor @eeckstein @nkcsgexi @rjmccall @jckarter @slavapestov I migrated the CODEOWNERS.txt info associated with you to the new format. This means that you will start receiving notifications about PRs that modify the files you own per the old specification. Please let me know if I missed anything, the old specification is outdated, or you do not want to receive notifications.
• @jckarter Is you being a code owner of "everything in Swift not covered by someone else" still accurate? If we are going to add a mapping for this, I propose to keep it as a comment until we reach a decent coverage rate, or else your mailbox might go ballistic 🙂.
• @adrian-prantl @artemcm @tshortli @compnerd Assigned you to a few more places you seem to have missed based on existing mappings.

[tshortli]

Do you by chance know who took charge of Serialization & Frontend files?

Serialization would be @xymus. I'll ask around about Frontend.

[akyrtzi]

@akyrtzi Do you want me to assign you to IDE, Index, SourceKit, and swift-ide-test files per the old CODEOWNERS.txt?

No, it should be @bnbarham

[tshortli]

@artemcm and I can be listed for Frontend.

[AnthonyLatsis]

@xymus Mapped you to serialization files. This means that you will start receiving notifications about PRs that modify these files. Please let me know if I missed anything or you if do not want to receive notifications.

[AnthonyLatsis]

All right, this seems good to go, so I have resurrected CODE_OWNERS.TXT to unblock the change. We will delete it once an updated code ownership governance lands on the website.

[AnthonyLatsis]

@swift-ci please smoke test