feat(prune): add ability to specify scopes via argument instead of flag (#6022)
### Description
As per @nathanhammond's suggestion, `--scope` should be an arg since it
is required.
UI change:
We now have help text for the `SCOPE` argument, wording isn't great, but
it matches the wording used for the overall prune command.
Clap will now complain about missing scopes for prune before our error
message in `prune.rs` where we check if [`scopes` is
empty](https://github.com/vercel/turbo/blob/main/crates/turborepo-lib/src/commands/prune.rs#L234C12-L234C12).
All existing usages of `turbo prune` will continue working just a
different error message:
Old:
```
[1 olszewski@chriss-mbp] /Users/olszewski/code/vercel/turborepo/turborepo-tests/integration $ turbo prune
Turbo error: at least one target must be specified
```
New:
```
[0 olszewski@chriss-mbp] /Users/olszewski/code/vercel/turborepo/turborepo-tests/integration $ turbo_dev prune
ERROR the following required arguments were not provided:
<SCOPE>...
Usage: turbo prune <SCOPE>...
For more information, try '--help'.
```
### Testing Instructions
Added new unit tests that use the arg variant and a new test to make
sure users can't mix the two ways to specify scope.
Closes TURBO-1357
Co-authored-by: Chris Olszewski <Chris Olszewski>