Decode a few keys in SourceKitLSPOptions that we missed before

We forgot to decode the following keys in the custom decode function, which meant that you couldn’t set them using SourceKit-LSP’s `config.json` file.
- `backgroundPreparationMode`
- `sourcekitdRequestTimeout`

In particular this meant that you couldn’t enable the `--experimental-prepare-for-indexing` preparation mode.

Add the necessary decoding functions in `release/6.0` and eliminate the custom decode logic entirely in `main`.

Author: ahoppen

Sources/SKCore/SourceKitLSPOptions.swift

@@ -379,6 +379,10 @@ public struct SourceKitLSPOptions: Sendable, Codable {
     )
     self.generatedFilesPath = try container.decodeIfPresent(String.self, forKey: CodingKeys.generatedFilesPath)
     self.backgroundIndexing = try container.decodeIfPresent(Bool.self, forKey: CodingKeys.backgroundIndexing)
+    self.backgroundPreparationMode = try container.decodeIfPresent(
+      String.self,
+      forKey: CodingKeys.backgroundPreparationMode
+    )
     self.experimentalFeatures = try container.decodeIfPresent(
       Set<ExperimentalFeature>.self,
       forKey: CodingKeys.experimentalFeatures
@@ -391,5 +395,9 @@ public struct SourceKitLSPOptions: Sendable, Codable {
       Double.self,
       forKey: CodingKeys.workDoneProgressDebounceDuration
     )
+    self.sourcekitdRequestTimeout = try container.decodeIfPresent(
+      Double.self,
+      forKey: CodingKeys.sourcekitdRequestTimeout
+    )
   }
 }