Add common patterns in code

[Michael Wayne Goodman]

The documentation has some common patterns defined, but it would also be useful to have some defined in code. But until there's a feature to add external definitions (e.g., a parameter to pe.compile() or some kind of import syntax within a grammar), these patterns will only be useful for non-parsed grammars using operator primitives.

More inspiration:

• Lark has some basics, like digits and whitespace (link)
• PEG.js includes unicode categories (link)