Support DocC generation in Swift Package Index (#205)

This allows documentation for our products to be generated and hosted automatically by [Swift Package Index](https://swiftpackageindex.com/swiftwasm/JavaScriptKit).

Author: MaxDesiatov

.spi.yml

@@ -0,0 +1,7 @@
+version: 1
+builder:
+  configs:
+    - documentation_targets: 
+      - JavaScriptKit
+      - JavaScriptEventLoop
+      - JavaScriptBigIntSupport