RT 68176 and 71705 -- add support for missing operators **=, x=, &&=, etc.

Changes to address https://rt.cpan.org/Public/Bug/Display.html?id=68176 and https://rt.cpan.org/Public/Bug/Display.html?id=71705 -- PPI not supporting some assignment operators like **=, x=, &&=, etc.

Adds all missing "augmented" assignment operators--e.g. "&&="--to PPI::Token::Operator.pm.
Adds tests for PPI::Token::Operator.pm.
Updates lexer test that relied on "||=" parsing as two operators.
Adds support for "x=" to PPI::Token::Word.

Note that the patch in RT 68176 does not fix "x=" or the lexer test, whereas this branch does both.

Word.pm looked like the best place to add x= support. Is there a better place?