Add workspace indexing progress notifications

[ewels]

Summary

Implements LSP $/progress notifications during workspace file indexing to provide visual feedback in editors/clients (fixes #148).

Written by Claude 🤖

Changes

• Add BiConsumer<Integer, Integer> progress callback parameter to ASTNodeCache.update()
• Report per-file progress using AtomicInteger for thread-safe counting in parallel stream
• Create ProgressNotification in LanguageService.update0() when indexing > 1 file
• Works for both initial workspace scan and batch file updates
• Uses workspace-specific token (indexing-{rootUri.hashCode()}) to support multiple workspaces

Details

Progress Flow Example

For a workspace with 60 files:

→ window/workDoneProgress/create { token: "indexing-12345678" }
← $/progress { token: "...", value: { kind: "begin", message: "Indexing 60 files...", percentage: 0 } }
← $/progress { token: "...", value: { kind: "report", message: "Indexing: 1 / 60 files", percentage: 1 } }
← $/progress { token: "...", value: { kind: "report", message: "Indexing: 2 / 60 files", percentage: 3 } }
...
← $/progress { token: "...", value: { kind: "report", message: "Indexing: 60 / 60 files", percentage: 100 } }
← $/progress { token: "...", value: { kind: "end" } }

Testing

• Progress notifications only appear when indexing more than 1 file
• Progress is reported per-file for accurate progress bars in clients
• finally block ensures progress.end() is always called even if exceptions occur

[ewels]

This PR allows me to build a progress bar whilst indexing a project, like this one:

Bigger pipelines have 60+ symbols and take quite a while, so it's nice to know that something is happening.