refactor: Update `workspace select` and `new` subcommands to use the arguments package for parsing arguments and flags (#38430)

* refactor: Move method `ValidWorkspaceName` and related const into arguments package, update references

This function is primarily used to validate arguments, but it's used once (in (m *Meta) Workspace) in a different context, so I've left the original function signature in the command package.

* feat: Add `workspace new`-related code to arguments package

* refactor: Update `workspace new` to use the arguments package when parsing arguments

Note that this changes the format of errors returned when arg parsing fails - this is now wrapped in a diagnostic so now starts with `\nError: ` and has an extra trailing newline.

* feat: Add `workspace select`-related code to arguments package

* refactor: Update `workspace select` to use the arguments package when parsing arguments

Note that this changes the format of errors returned when arg parsing fails - this is now wrapped in a diagnostic so now starts with `\nError: ` and has an extra trailing newline.