chore: support $eval (#9)

* chore: support

* chore: bump go version

* chore: replace reflect.PtrTo with PointerTo

* chore: add tests for $eval

fix: (null) return correct value (#8)

fix: null handling (#5)

* test: add some null handling test cases

* fix: correct support of null

blues/jsonata-go v1.5.4

pure library

package change

v1.5.4 with go 1.20.3