Function parseSelector

Parse a UCAN policy selector into segments.

Consumes the whole string and throws a TypeError on anything that is not valid selector syntax. Supported forms:

• . identity
• .name dotted field name, where name matches [A-Za-z_][A-Za-z0-9_]*
• ["any string"] unambiguous field name, with JSON string escapes
• [] collection values
• [n], [-n] list index
• [a:b], [a:], [:b] list slice
• a trailing ? on any segment (repeated ? collapse into one)

A selector MUST start with . and MUST NOT contain ...