feat: provide ability to overwrite feature flags in esm-bundler builds

e.g. by replacing `__VUE_OPTIONS_API__` to `false` using webpack's
`DefinePlugin`, the final bundle will drop all code supporting the
options API.

This does not break existing usage, but requires the user to explicitly
configure the feature flags via bundlers to properly tree-shake the
disabled branches. As a result, users will see a console warning if
the flags have not been properly configured.

packages/shared/src/index.ts

@@ -146,3 +146,20 @@ export const toNumber = (val: any): any => {
   const n = parseFloat(val)
   return isNaN(n) ? val : n
 }
+
+let _globalThis: any
+export const getGlobalThis = (): any => {
+  return (
+    _globalThis ||
+    (_globalThis =
+      typeof globalThis !== 'undefined'
+        ? globalThis
+        : typeof self !== 'undefined'
+          ? self
+          : typeof window !== 'undefined'
+            ? window
+            : typeof global !== 'undefined'
+              ? global
+              : {})
+  )
+}