`toQuantity()` ignores its optional `unit` parameter

[luisfabib]

The FHIRPath spec defines toQuantity() with an optional unit argument:
toQuantity([unit : String]). The parser correctly accepts 0 or 1 arguments, but
ToQuantity.evaluate() never reads or applies the argument — it is silently dropped.

Additionally, the parser does not star-unpack the argument list when constructing the
ToQuantity node, so the argument is lost before it even reaches evaluate().

Expected behaviour

'1.5'.toQuantity('mg') → 1.5 'mg'
'1.5'.toQuantity() → 1.5 '1' (dimensionless)

Actual behaviour

'1.5'.toQuantity('mg') → 1.5 '1' (unit argument ignored)

Suggested fix

1. In parser.py: pass the optional unit string argument when constructing ToQuantity.
2. In conversion.py: apply the unit argument in evaluate(), overriding the parsed unit
   if one is provided.