#2180: Add *_PLUGINS_EXTRA variable to install extra IDE plugins per user

[Paras14]

This PR fixes #2180

Implemented changes:

• Added the tool-specific variable «TOOL»_PLUGINS_EXTRA (e.g. VSCODE_PLUGINS_EXTRA=eslint,cpp-tools,docker) allowing a user to activate additional plugins without modifying the shared project settings.
• Values refer to the filename of the plugin properties file without the .properties extension, not to the id inside it. Unresolvable entries are skipped with a warning so a single stale entry cannot break the installation of the remaining plugins.
• Implemented in PluginBasedCommandlet.getPlugins() rather than in a specific IDE commandlet, so it applies to Eclipse, VSCode and all IntelliJ-based IDEs. Plugins activated this way are reinstalled automatically when plugins are purged, unlike plugins installed manually in the IDE.
• Documented in documentation/plugin.adoc and documentation/variables.adoc.
• Note: removing an entry from «TOOL»_PLUGINS_EXTRA does not uninstall the plugin (the marker file survives). This is documented; resetting is done via --force-plugin-reinstall.

---

Testing instructions

1. Build and activate this branch locally: export IDE_ROOT=/your/ide/root && ./build-local-dev.sh (revert afterwards with ideasy upgrade --mode=stable).
2. In any IDEasy project, run ide vscode and note a plugin logged as Omitting installation of inactive plugin icons (...), plus the plugin count in the Install plugin ... (n/m) steps.
3. Add to $IDE_HOME/conf/ide.properties:
   VSCODE_PLUGINS_EXTRA=icons
4. Run ide vscode again. Expected: Start: Install plugin icons (n/m) with m one higher than in step 2, and the extension present under $IDE_HOME/plugins/vscode. The shared settings/ directory is untouched (git status inside it stays clean).
5. Replace the value with the plugin's id instead of its filename, e.g. VSCODE_PLUGINS_EXTRA=vscode-icons-team.vscode-icons, and run again. Expected: a warning that the plugin is undefined, and icons is not installed, the variable takes filenames only.
6. Set VSCODE_PLUGINS_EXTRA=icons,typo-does-not-exist and run again. Expected: one warning for the unknown entry, icons installs anyway.
7. The same works for other IDEs, e.g. INTELLIJ_PLUGINS_EXTRA=watchers (close IntelliJ first, IDEasy calls idea64.exe to install plugins).

---

Checklist for this PR

Make sure everything is checked before merging this PR. For further info please also see
our DoD.

• When running mvn clean test locally all tests pass and build is successful
• PR title is of the form #«issue-id»: «brief summary» (e.g. #921: fixed setup.bat). If no issue ID exists, title only.
• PR top-level comment summarizes what has been done and contains link to addressed issue(s)
• PR and issue(s) have suitable labels
• Issue is set to In Progress and assigned to you or there is no issue (might happen for very small PRs)
• You followed all coding conventions
• You have added the issue implemented by your PR in CHANGELOG.adoc unless issue is labeled
  with internal
• You have formulated clear instructions on how to test your contribution under "Testing instructions"