Add Parse extension for conversion initializer

[coenttb]

Summary

• Adds a convenience initializer to Parse that accepts a conversion and applies it to the rest of the input
• This extension was originally in the URLRouting library but is better placed here as it's not specific to URL routing
• All tests pass with this change

Test plan

• All existing tests continue to pass
• The new extension compiles successfully and integrates with existing functionality

[stephencelis]

@coenttb A few things:

1. Can you please review your diffs before submitting? My guess is you didn't intend to contribute a funding.yml or dependabot.yml in this PR. I mention this because it's happened a couple times before 😄
2. It's fine to use AI to workshop code or summaries before submitting, but please give a final pass in your own code/words to make it easier for us to understand the purpose of your PR. While you mention that this initializer exists in URLRouting, you make no mention of why you need it in Parsing (e.g. what problem it solves for you, a reminder to us of what problem it is solving in URLRouting, an example of why you need it, etc.).

[stephencelis]

(For some reason I got a notification of this old PR today, but we probably shouldn't have lost track of it for so long. Sorry!)

[coenttb]

Hi Stephen, very valid feedback, which I take very seriously. I do regret these issues.
Would you prefer I close this PR, or take another pass at it?

[stephencelis]

@coenttb Up to you! If you think this PR is something that would add value to the library, please do take another pass. It's hard for me to know without you motivating it 😄