Add support for workspace symbols (project-wide symbol search)

**I'm submitting a...**

- [ ] Bug report.
- [x] Feature request.

## Feature request ##

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**Describe the problem you're trying to solve.**

The new outline tree greatly speeds up navigation inside a file. Navigation could be improved even further by adding the tokens that populate the outline tree as project symbols. This way, we could jump to them inside and across files with the "project symbols: toggle" command (CTRL+ALT+F on my system). This would behave roughly like the "Go To Function..." command in the built-in Godot editor.