Add a `wp user exists` subcommand

[kkmuffme]

Feature Request

• [x] Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

Using wp user get writes to stderr when the user doesn't exist, which makes it porcelain, since it needs to be parsed and processed to be able to distinguish between user doesn't exist and unrelated errors.

Describe the solution you'd like

There should be a command like wp user exists to check whether a user exists or not, returning 0 on success, 1 if it doesn't exist and not write anything to stderr in that case. Like wp plugin is-active which does this for plugins.

[swissspidy]

Sounds reasonable 👍 There are already similar commands such as wp post exists and wp comment exists.

We could consider adding an exists command for all entities that have a get command already, for consistency.