Added `null` and `undefined` as built-in values
Added `+=`, `-=`, `*=`, `/=` shorthand update operators (x |= . op [filter])
Bugfix: comparing with a filter that has no results

- Fixed a bug where using a comparison operator on a filter that returned no results would result in no comparison being performed. An unfound filter now resolves to `undefined` in a comparison.
Bugfix: wrong input when using a filter in a variable's filter

- Fixed a bug where filtering a variable using a filter as value would result in this filter using the variable's value as input instead of the correct input.
Added `===` comparison operator
Parser tool: small updates