[core] Stabilize tree export API

[oowekyala]

The API around TreeRenderer has been @Experimental since it was introduced in PMD 6.21.0 (#2216). I think it's been rather stable and we could promote it to a stable API.

[jsotuyod]

#4059 does refactor its implementation (splitting JCommander CLI interface, configuration and actual logic into 3 separate classes), and adds debug / verbose flags, but probably after that we can reassess its maturity.

[adangel]

TODO:

• check net.sourceforge.pmd.util.treeexport.TreeExportConfiguration#setReportFile(java.nio.file.Path) is not used. Should be probably used by net.sourceforge.pmd.cli.commands.internal.TreeExportCommand. found in [core] Refactor CPD #4397.
  • postponed via [cli] CPD/TreeExport: support --report-file parameter #4653
• TreeExportConfiguration inherits from AbstractConfiguration - there is "relativizeRoots", which probably doesn't make sense for that tool.
• Update example on https://docs.pmd-code.org/latest/pmd_devdocs_experimental_ast_dump.html - it looks like it's still the old AST from PMD 6.x