Port the Jython completion engine from the (now-retired) script-editor-jython
repo into this scripting plugin, so Jython provides both its ScriptLanguage and
its CodeCompleter while depending only on scijava-common + jython — no Swing or
RSyntaxTextArea.

- JythonCodeCompleter: a CodeCompleterPlugin that derives the line context from
  a CompletionRequest and returns neutral Completions + a ParameterChoices.
- JythonAutoCompletions: the analysis engine, emitting neutral
  org.scijava.script.complete.Completion (no RSTA), with auto-imports expressed
  as Completion.TextEdit and parameter assistance via ParameterChoices.
- JythonScriptParser/Scope/DotAutocompletions* and an internal CompletionText:
  unchanged AST logic, with class discovery/javadoc via scijava-common's
  ClassIndex.
- The legacy RSTA glue (LanguageSupportPlugin, AutocompletionProvider,
  AutoCompletion, FunctionCompletion) is dropped; syntax highlighting moves to
  script-editor. The hardcoded ImageJ ij.dir scan is replaced by locating the
  jython jar on the classpath.

Adds an antlr-runtime dependency (Jython AST) and a temporary scijava-common
version override until a release pins the new completion SPI.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Still need a version newer than pom-scijava though