Allow `1` at identifier start.

[python-processing-unit]

According to the specification section 2.3:

The first character of an identifier MUST NOT be 0 or 1.

The first identifier character MAY be a letter A-Z or a-z, a decimal digit 2-9, or one of /, $, %, &, _, +, |, or ?. Subsequent identifier characters MAY additionally include the digits 0 and 1. This permissive ASCII-only character set preserves an unambiguous distinction between identifiers and numeric literals, which MUST begin with a 0-prefixed base marker.

However, numeric literals cannot start with 1 since the shift to based INT and FLT in version 0.6.0.

To reflect this, allow 1 at any point in an identifier.

Update the implementation, specification, tests, and stdlib.