build: use SwiftBuild for xtool workspace LSP (#282)

## What does this PR do?

Configure SourceKit-LSP to use SwiftBuild when working on xtool itself.
The legacy build system has trouble with xcframeworks.

## How was it tested?

Confirmed that it fixes highlighting of error diagnostics

## AI tool usage

How much of this PR was AI-assisted? (check one)

- [x] **0** - No AI was used to write code
- [ ] **1** - I was assisted by AI. I reviewed the finished result.
- [ ] **2** - I set the AI going and left it to it; nobody has read the
result - no review, or AI review only

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.sourcekit-lsp/config.json

@@ -0,0 +1,7 @@
+{
+    "swiftPM": {
+        "buildSystem": "swiftbuild",
+        "scratchPath": ".build/index-build"
+    },
+    "backgroundIndexing": true
+}