Support for the `throttle` DSL was initially added to
`bump-formula-pr` but apparently never added to `bump-cask-pr`, which
I only discovered after trying to throttle a cask. This adds basic
support for handling a `throttle` value in `bump-cask-pr` using the
same approach as `bump-formula-pr`.

One thing to note is that this currently only supports simple casks
with one top-level `version` and `livecheck` block. Other parts of
bump have issues with arch- and OS-specific version bumps and I have
some in-progress work to try to improve that, so I can't make any
guarantees beyond simple casks. I will likely revisit how versions are
handled in `check_throttle` as part of that work, so I borrowed the
simple logic for selecting a version from the `BumpVersionParser`
object for the time being (i.e., ARM -> general but also falling back
to Intel here).

The two casks that I know of that we're currently manually throttling
using a `strategy` block aren't able to be supported by this initial
implementation. `bettertouchtool@alpha` doesn't have a patch version
and instead throttles based on the `version.csv.second` (build) value.
`commander` doesn't create releases for every version number, so
requiring a version to be divisible by the throttle number would cause
the cask to skip many more versions than intended. I intend to handle
these situations in follow-up PRs by adding options to the `throttle`
DSL to dictate what version part to check and to disable the rounding
behavior respectively but this is a start.