Migrate skip checks to --EXTENSIONS--, p1

For rationale, see https://github.com/php/php-src/pull/6787

Extensions migrated in this part:
* bcmath
* bz2
* calendar
* c

Migrate skip checks to --EXTENSIONS--, p1

For rationale, see https://github.com/php/php-src/pull/6787

Extensions migrated in this part:
* bcmath
* bz2
* calendar
* com_dotnet
* ctype

Closes GH-6797.

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Promote some warnings in BCMath to Errors

Also do a bit of refactoring at the same time.

Closes GH-6105

Fix test

Test fixes

Fix #78878: Buffer underflow in bc_shift_addsub

We must not rely on `isdigit()` to detect digits, since we only support
decimal ASCII digits in the following processing.