Fix TypeScript errors from terriajs-cesium workspace package @ts-check files

Cesium started adding `@ts-check` to their JS source files. Because
terriajs-cesium can be a yarn workspace package symlinked into node_modules,
TypeScript resolves the symlink to the real path outside node_modules and
reports errors from those files.

preserveSymlinks: true keeps the symlink path intact so TypeScript treats
the cesium source as library code and suppresses error reporting.

Author: zoran995

tsconfig.json

@@ -26,6 +26,7 @@
       "../../../node_modules/",
       "../../../node_modules/@types/"
     ],
+    "preserveSymlinks": true,
     "types": ["terriajs-cesium", "@testing-library/jasmine-dom"]
   },
   "include": ["./lib/**/*", "./test/**/*", "./buildprocess/**/*"]