Write scripts to handle when Flutter ships a new revision

[eseidel]

We should just do this as part of updating to 3.7.9 tomorrow.

Updating to 3.7.9 is no longer as pressing since we now vend our own Flutter, but it's still something we need to do soon.

[eseidel]

3.7.10 came out today. So scripts will definitely be needed if we're doing this every week. 🤣

[eseidel]

Wrote up docs for now: https://github.com/shorebirdtech/shorebird/blob/main/FORKING_FLUTTER.md
Will write up a script next time we have to do this.

[eseidel]

It's not fully automated yet, but at least getting the revisions is and generating the commands:
#336