Add Jenkins Symbols autocomplete

[janfaracik]

Very rough still + still exploratory, but thought I'd post to get thoughts.

Looking to add autocomplete for Jenkins Symbols to make the developer experience cleaner.

Current thinking is to offer suggestions from the project, core and the ionicons API (if installed).

Testing done

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[duemir]

If it is a generic system that relies on certain file types placed in directories with specific names, it might be possible to use Intellij indexes to look them all up, e.g. https://plugins.jetbrains.com/docs/intellij/file-based-indexes.html#file-name-index

If it makes things simpler, we can also add Maven plugin as a dependency. For example, https://plugins.jetbrains.com/docs/intellij/xml-dom-api.html mentions Maven plugins as a reference, so model of the maven project might already be exposed in a relatively easy to get way.