[Sed] Add Completion Spec

[ankithm28]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Completion Spec
What is the current behavior? (You can also link to an open issue here)
#102
What is the new behavior (if this is a feature change)?

Additional info:

[withfig-bot]

Overview

dev/sed.ts:

Info:

[withfig-bot]

Hello @monkeyank,
thank you very much for creating a Pull Request!
Here is a small checklist to get this PR merged as quickly as possible:

• Do all subcommands / options which take arguments have the arg property (arg: {})?
• Are all options modular? E.g. a -u -x instead of -aux
• Have all other checks passed?

Please add a 👍 as a reaction to this comment to show that you read this.

[QuiiBz]

I'm not familiar with the Sed CLI but this looks fine to me!

[QuiiBz]

LGTM!