Promote "undefined array key" notice to warning

This implements the last remaining part of the
https://wiki.php.net/rfc/engine_warnings RFC.

Closes GH-5927.

Implement 'Saner Numeric Strings' RFC:

RFC: https://wiki.php.net/rfc/saner-numeric-strings

This removes the -1 allow_error mode from is_numeric_string functions and replaces it by

Implement 'Saner Numeric Strings' RFC:

RFC: https://wiki.php.net/rfc/saner-numeric-strings

This removes the -1 allow_error mode from is_numeric_string functions and replaces it by
a trailing boolean out argument to preserve BC in a couple of places.

Most of the changes can be resumed to "numeric" strings which emitted a E_NOTICE now emit
a E_WARNING and "numeric" strings which emitted a E_WARNING now throw a TypeError.

This mostly affects:
- String offsets
- Arithmetic operations
- Bitwise operations

Closes GH-5762

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