Add TypeScript definition file

Author: Jamie Wong

index.d.ts

@@ -0,0 +1,27 @@
+declare module 'browserstack-local' {
+  interface Options {
+    key: string
+    verbose: boolean
+    force: boolean
+    only: string
+    onlyAutomate: boolean
+    forceLocal: boolean
+    localIdentifier: string
+    folder: string
+    proxyHost: string
+    proxyPort: string
+    proxyUser: string
+    proxyPass: string
+    forceProxy: boolean
+    logFile: string
+    parallelRuns: string
+    binarypath: string
+    [key: string]: string | boolean
+  }
+
+  class Local {
+    start(options: Partial<Options>, callback: () => void): void
+    isRunning(): boolean
+    stop(callback: () => void): void
+  }
+}