Major selector enhancements

[mtdowling]

This commit adds several new features to selectors, updates its grammar, and rewrites the parser to be more adaptable to change.

• Add support for numeric attribute comparators. You can match values that are parsed as numbers using >, >=, <, <=. This can be used to find out things like [trait|httpError >= 500].
• Add support for != to check if an attribute exists but isn't equal to the given value.
  The "!=" attribute comparator is used to find shapes that have an attribute, but the attribute is not equal to a given value. For example, the following selector: [trait|error!=client], is a simplification of the following:
  [trait|error]:not([trait|error=client])
• Add support for CSV values in attribute selectors. This enables checking if a resolved attribute value matches one or more comparison values, allowing for things like [trait|httpError = 500, 501].
• Fix "~=" in docs to properly reference "^=". The docs previously documented the wrong comparator.
• Adds support for numbers in attribute selectors. You no longer need to quote numbers in selector attribute values.
• Shape IDs with members must be quoted. This is to remove a parsing ambiguity with the $= comparator.
• Add support for selecting nested trait properties. Nested trait properties can be selected using a "|" delimited syntax. Values or lists and objects can be "projected" using the special (values) property. Keys of objects can be projected using the special (keys) property.
• pseudo-properties are now formalized in the grammar and parser as"(" identifier ")", allowing us to add more in the future as needed.
• This commit updates selectors to tolerate unknown functions and renames the ":each" selector to ":is" to match the CSS4 name of ":is", which is the same functionality. Support for ":each" is still present and functions as an alias.