Add ability to publish to local Maven repo

[bannmann]

This PR allows developers to publish RR to their local Maven repository directory.

In another (locally built) project, one can then declare a dependency Maven/Gradle dependency on de.bottlecaps.rr:rr-lib:1.63.

Note: my end goal is to create an RR Maven plugin. For this I will need additional (mostly harmless) changes, but I'd like to submit separate PRs for those.

[bannmann]

Let me know if you prefer de.bottlecaps.railroad as the group ID (or just edit the PR accordingly). I don't care either way. Usually I'd go with the root Java package name, but the "official product name" seems to be more "RR" than "Railroad".

[GuntherRademacher]

No objection.

In my test configuration (Java 15, Gradle 7.2, Buildship 3.1.5) I had to explicitly exclude xml-apis, like this:

testImplementation ('de.bottlecaps.rr:rr-lib:1.63') { exclude module: 'xml-apis' }

in order to avoid complaints about duplicate packages javax.xml.parsers and others, which are present in xml-apis as well as in the JRE.