Migrate code from com.fasterxml.jackson.core to tools.jackson.core

It seems we may need to move from version 2.x to version 3.1 of the jackson core tools. This is not a direct drop-in replacement as the package name has changed and the ObjectMapper approach also changed. This would generate a new major version of JSONata4Java.

Document retained Jackson 2 behaviors and known issues (#430)

Document Maven build-warning cleanup in migration design doc (#430)

Follow-up on the branch (jackson-3-migration, PR #431): cleaned up three pre-existing Maven build warnings in pom.xml that were surfacing on every build:

bnd private-reference (×2): added com.api.jsonata4java.expressions.regex to Export-Package (its types are exposed in the public API of the already-exported com.api.jsonata4java / com.api.jsonata4java.expressions packages).
additionalClasspathElements unknown (×2): removed the empty element from maven-compiler-plugin (not a valid parameter).
excludes unknown: removed from the maven-assembly-plugin single goal (not a valid parameter; was silently ignored).

mvn clean package now builds with these warnings gone (BUILD SUCCESS). A fourth warning — gpg.passphrase deprecated — comes from a developer's local ~/.m2/settings.xml, not this repo, and is resolved locally by relying on gpg-agent. Recorded in docs/specs/Jackson-3-Migration-Design.md.