use/support new Conditional query filter

[tyrasd]

see https://wiki.openstreetmap.org/wiki/Overpass_API/Overpass_QL#Conditional_query_filter_.28if:.29

• investigate if it makes sense to use this new syntax in certain situations
• implement more operators which were previously not easily possible

[mmd-osm]

A few topics come to mind:

• Numeric comparison, e.g. maxspeed > 30: (if: (is_number(t[maxspeed]) && t[maxspeed] > 30))
• tag count
• node count
• is_closed()