Don't transpile to ES5

This is meant as a discussion for a future major version of Apollo
Client.

Right now the published package is using TypeScript's downleveling emit
directives to compile to ES5. This *irreversibly* bloats the package
with cruft that most users won't need. Additionally, it affects
performance since native ES features are simulated using clunky
workarounds. This is especially noticeable with async/await where any
async functions compile into a state machine. This also affects nullish
coalescing (`??`), optional chaining (`?.`), arrow functions, and a lot
more.

My suggestion is to publish this package using the code as it was
written and allow the user's bundler to perform transpilation and
minification as configured.

This reduces the size of `apollo-client.min.cjs` from 120kb to 104kb
which is pretty good for a one-line change.

.changeset/calm-pears-relate.md

@@ -0,0 +1,5 @@
+---
+"@apollo/client": major
+---
+
+Don't downlevel published package

tsconfig.json

@@ -11,7 +11,7 @@
     "sourceMap": true,
     "declaration": true,
     "declarationMap": true,
-    "target": "es5",
+    "target": "esnext",
     "module": "es2015",
     "esModuleInterop": true,
     "experimentalDecorators": true,