Plural rules: updates for UTS #35 Rev 33

* New operands i, v, w, f, t
* New operators =, !=, %
* Ignore "samples", which are basically unit tests embedded in rule
  specifications
* Ignore the new "other" rules, which have an empty condition. It
  doesn't really makes sense to parse them, since the empty condition
  means special handling should be done in the caller, it is not
  equivalent to an unconditional true or false.
* Trailing zero support requires that the input number be a string.
  Documented this.
* Fixed some comments
* Added test cases for new features

Bug: 56931
Change-Id: I96986c0c664f785e75b0a4ced2ec9e37b72681c1
Merge "Plural rules: updates for UTS #35 Rev 33"