[64-bit ints] Add SCRIPT_ENABLE_63_BIT_INTS flag

Summary:
Add the flag (always disabled for now) to run the script interpreter with support for 63+sign-bit integers.

This flag is only needed for activation logic, and after 63+sign-bit integers have been enabled, we can remove the flag and pretend the chain has always had 63+sign-bit integers.

Note that parsing and disassembling 63+sign-bit integers isn't supported yet, which will be added in another diff. Therefore, script_tests.json also wasn't updated.

Partial backport of lotusd#71.

Depends on D17001.

Test Plan: ninja check

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: shbaismail

Differential Revision: https://reviews.bitcoinabc.org/D17300