Update typescript to 3.9 and enable skipLibCheck

TS 3.9 offers a few speed improvements. Nothing huuuuuge, but it shaves 3
seconds of our build and type-check commands.

Enabling skipLibCheck (as set by our parent TS configs) meanwhile saves
an additional 10 seconds as we don't bother checking TS declaration files
that we don't use. Recently TS's init scripts were updated so now this
is set to true in newly creacted configs so I'm now more confident in
following suit.

On Master: type-check took ~38s, build took ~52s
This commit: type-check took ~24.5s, build took ~37s

UNRELEASED.md

@@ -12,8 +12,12 @@
 
 ### Development workflow
 
+- Improved speed of type-check and build by enabling TypeScript's `skipLibCheck` option ([#2981](https://github.com/Shopify/polaris-react/pull/2981))
+
 ### Dependency upgrades
 
+- Updated TypeScript to 3.9.2 ([#2981](https://github.com/Shopify/polaris-react/pull/2981))
+
 ### Code quality
 
 ### Deprecations

package.json

@@ -178,7 +178,7 @@
     "shx": "^0.3.2",
     "storybook-chromatic": "^3.4.1",
     "svgo": "^1.3.0",
-    "typescript": "~3.8.3"
+    "typescript": "~3.9.2"
   },
   "peerDependencies": {
     "react": "^16.8.6",

tsconfig.json

@@ -11,8 +11,7 @@
     // strictFunctionTypes is implictly enabled by strict mode but
     // we're not yet compliant, so disable for now
     "strictFunctionTypes": false,
-    "importHelpers": true,
-    "skipLibCheck": false
+    "importHelpers": true
   },
   "include": ["./config/typescript", "./src", "./tests", "./playground"]
 }

yarn.lock

@@ -17846,15 +17846,10 @@ typedarray@^0.0.6:
   resolved "https://registry.yarnpkg.com/typedarray/-/typedarray-0.0.6.tgz#867ac74e3864187b1d3d47d996a78ec5c8830777"
   integrity sha1-hnrHTjhkGHsdPUfZlqeOxciDB3c=
 
-typescript@^3.7.2:
-  version "3.7.3"
-  resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.7.3.tgz#b36840668a16458a7025b9eabfad11b66ab85c69"
-  integrity sha512-Mcr/Qk7hXqFBXMN7p7Lusj1ktCBydylfQM/FZCk5glCNQJrCUKPkMHdo9R0MTFWsC/4kPFvDS0fDPvukfCkFsw==
-
-typescript@^3.8.0-dev.20200111, typescript@~3.8.3:
-  version "3.8.3"
-  resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.8.3.tgz#409eb8544ea0335711205869ec458ab109ee1061"
-  integrity sha512-MYlEfn5VrLNsgudQTVJeNaQFUAI7DkhnOjdpAp4T+ku1TfQClewlbSuTVHiA+8skNBgaf02TL/kLOvig4y3G8w==
+typescript@^3.7.2, typescript@^3.8.0-dev.20200111, typescript@~3.9.2:
+  version "3.9.2"
+  resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.9.2.tgz#64e9c8e9be6ea583c54607677dd4680a1cf35db9"
+  integrity sha512-q2ktq4n/uLuNNShyayit+DTobV2ApPEo/6so68JaD5ojvc/6GClBipedB9zNWYxRSAlZXAe405Rlijzl6qDiSw==
 
 ua-parser-js@^0.7.18:
   version "0.7.19"