This will need a rebase once #652 is merged. Only the latest six commits are really part of this PR.

• Closes Loop detection could be done in message/term reference instead of pattern (?) #277
  As suggested in the issue, loop detection is moved to message & term resolution. It now tracks expressions rather than patterns.

• Closes @fluent/bundle how to use FluentType.toString #426
  While the type of Scope is indeed public, it's here made an optional argument of all FluentType.p.toString() implementations.

• Closes Accept boolean variables in Localized component #543
  Boolean values are supported as argument values; internally, they are handled as strings, serialised as either "true" or "false".

• Closes Automatic Eastern Arabic numerals in Arabic translations?H #583
  Formatting with ar as the locale continues to default to Latin digits, but the numbering system can be customized for all messages via a locale extension (e.g. ar-u-nu-arab), or for a specific placeholder by wrapping the number in a FluentNumber and specifying its numberingSystem option value. Tests are added to validate this.

• Closes Request Feature: Support prefix identifier for message #624
  The parser is moved to a function outside the FluentResource class, which effectively makes it private within that module. It's also slightly optimised, e.g. removing repetitions of testing the same string with the same regexp.

• Additionally, a selector with no variants is now treated as a syntax error, as it should be.