Update default target from safari13 to safari14.

From vitejs#9062, it sounds like the goal is to support ES2020 by default. However, the browser list was set to `safari13`, which means that big ints (an ES2020 feature) produced an error by default.

This PR updates from `safari13` to `safari14` (released September 2020) to support that.

Thanks for any consideration of this change!

packages/vite/src/node/constants.ts

@@ -18,7 +18,7 @@ export const ESBUILD_MODULES_TARGET = [
   'edge88',
   'firefox78',
   'chrome87',
-  'safari13' // transpile nullish coalescing
+  'safari14'
 ]
 
 export const DEFAULT_EXTENSIONS = [