Investigation - Lexer - Support Unicode Identifiers

Currently, the lexer only matches [a-z_]+[a-zA-Z0-9_]* for identifiers (terms). However, most modern languages support the full set of Unicode word characters.

It its important to remember that Orbit is opinionated about identifiers, requiring them to begin with 1 or more lowercase letters, to distinguish them from type identifiers, which are always capitalised.

Before we can support Unicode identifiers, we need to determine what we consider a lowercase character. Do we say any character outside of ASCII a-z can be used in place of a lowercase? Or, do we define a strict subset of Unicode characters that may be used as lowercase?