Add VS Code workspace config

Summary:
## Summary

Adds and recommends use of `react-native.code-workspace` file when using VS Code. This disables the built-in TypeScript Language Service for `.js` files, recommends extensions, enables `formatOnSave`, and configures Flow language support.

**Motivation**

This is a DevX benefit for **React Native contributors** using open source VS Code — in particular to help with recent/trivial papercuts in PRs such as inserting a final newline in files (configured by EditorConfig).

**Recommended extensions**

NOTE: The recommended extensions list is currently minimal — happy to extend this now or in future, but let's aim to keep these conservative.

- Flow — language support
- EditorConfig — formatting based on `.editorconfig`, all file types
- Prettier — formatting for JS* files
- ESLint — linter for JS* files
- Kotlin — language support

**Why `react-native.code-workspace`?**

`.code-workspace` files have slight extra behaviours over a `.vscode/` directory:
- Allows user to opt-in or skip.
- Allows double-click launching from file managers.
- Allows base folder (and any subfolders in future) to be opened with local file tree scope (useful in fbsource!)
- (Minor point) Single config file over multiple files.

https://code.visualstudio.com/docs/editor/workspaces

Changelog: [Internal]

## Test plan

Aganst a new unconfigured copy of Visual Studio Code Insiders.

**Without workspace config**

❌ `.js` files raise errors by default (built-in TypeScript language service)

 {F1478195672} 

❌ When using the Flow VS Code extension, the wrong version (global) of Flow is used.

**With workspace config**

✅ Workspace config is suggested when folder is opened in VS Code

 {F1478194795} 

✅ Dialog is shown on workspace launch with recommended VS Code extensions

{F1478196003} 

✅ Built-in TypeScript Language Service is disabled for `.js` files
✅ Flow language support is configured correctly against `flow` version in `package.json`

{F1478291085}
{F1478200649}

Differential Revision: D55698495

react-native.code-workspace

@@ -0,0 +1,21 @@
+{
+	"folders": [
+		{
+			"path": "."
+		}
+	],
+	"extensions": {
+		"recommendations": [
+			"dbaeumer.vscode-eslint",
+			"editorconfig.editorconfig",
+			"esbenp.prettier-vscode",
+			"flowtype.flow-for-vscode",
+			"mathiasfrohlich.Kotlin"
+		],
+	},
+	"settings": {
+		"editor.formatOnSave": true,
+		"flow.pathToFlow": "${workspaceFolder}/node_modules/.bin/flow",
+		"javascript.validate.enable": false
+	}
+}