Have `expr` at higher precedence.

This is so that better error messages are generated.
Added percentage operator (5% => 0.05).
Color percentage symbol as an operator.
Added "UNSAVED" indicator.
Separate builtins from eval scope.
Added abs function.
Updated help popup.
Removed lang playground.
Scrollbar doesn't overlap topbar.
Fix last line giving erroneous results sometimes.
Fix javascript's floating point inaccuracies.
Make popups dropdown from behind the header.
Removed unused style.
Added fix decimal points' config UI.
Settings are now saved and loaded.
Realtime updating of fix precision option change.