Support unions of filters

[glyn]

https://cburgmer.github.io/json-path-comparison/results/union_with_filter.html

[glyn]

This is deferred since:

• It doesn't seem particularly useful. Filters can already be combined using ||, so unions of filters are only really required when combining a filter with ranges/subscripts.
• Only four implementations in the comparison implement unions of filters, so it seems unlikely to become the consensus.
• The syntax change is extensive given how array accesses are currently parsed.
• The implementation change is quite large given how array accesses are currently implemented.