displayable.toDisplayString: number formatting for primitives

[s-leroux]

toDisplayString falls back to String(obj) for primitives; no way to apply alignment/number format like object branch.

Location: src/displayable.mts:103

Code:

// XXX Shouldn't we allow number formatting specifiers?
return String(obj);

Possible approach: Plumb DisplayOptions (or a narrowed number format) into the primitive path, reusing the same mini-language as format() / amount formatters where appropriate.

[s-leroux]

At the very least, toDisplayString(number, format) should be supported.