Add schemas package

Creates a new package to hold the block.json and theme.json files that
can then be synced to the SchemaStore. We want a package within
Gutenberg so we can use the schemas internally to validate package files
such as blocks in the block-library as well as generate documentation.

Closes #35927

packages/schemas/.npmrc

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+package-lock=false

packages/schemas/CHANGELOG.md

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+<!-- Learn how to maintain this file at https://github.com/WordPress/gutenberg/tree/HEAD/packages#maintaining-changelogs. -->
+
+## Unreleased
+
+## 2.2.0 (2021-07-21)
+
+### Enhancement
+
+-   Ensure that the warning for a given message is logged only once.
+
+## 2.1.0 (2021-05-20)
+
+## 2.0.0 (2021-05-14)
+
+### Breaking Changes
+
+-   Drop support for Internet Explorer 11 ([#31110](https://github.com/WordPress/gutenberg/pull/31110)). Learn more at https://make.wordpress.org/core/2021/04/22/ie-11-support-phase-out-plan/.
+-   Increase the minimum Node.js version to v12 matching Long Term Support releases ([#31270](https://github.com/WordPress/gutenberg/pull/31270)). Learn more at https://nodejs.org/en/about/releases/.
+
+## 1.4.0 (2021-03-17)
+
+## 1.1.0 (2020-04-15)
+
+### New feature
+
+-   Include TypeScript type declarations ([#18942](https://github.com/WordPress/gutenberg/pull/18942))
+
+## 1.0.0 (2020-02-04)
+
+Initial release.

packages/schemas/README.md

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+# Schemas
+
+The collection of schemas used in WordPress, including the `theme.json` schema, and `block.json` schemas.
+
+## Installation
+
+Install the module
+
+```shell
+$ npm install @wordpress/schemas --save-dev
+```
+
+## Usage
+
+JSON schemas are used by code editors to offer tooltips, autocomplete, and validation. To use in your JSON file, add the `$schema` property to the top of the file with the value of the schema URL.
+
+For example, in your `block.json` file you would add:
+
+```json
+{
+	"$schema": "https://raw.githubusercontent.com/WordPress/gutenberg/trunk/packages/schemas/json/block.json"
+}
+```
+
+## SchemaStore.org
+
+[SchemaStore.org](https://schemastore.org) is an open collection of schemas, including the WordPress `theme.json` and `block.json` schemas. Any changes to schemas in this package should be mirror there.
+
+To update on SchemaStore, create a PR at their repository [SchemaStore/schemastore](https://github.com/SchemaStore/schemastore/). The files can be found in their reposiory at:
+
+-   block.json: `src/schemas/json/block.json`
+-   theme.json: `src/schemas/json/theme-v1.json`
+
+<br/><br/><p align="center"><img src="https://s.w.org/style/images/codeispoetry.png?1" alt="Code is Poetry." /></p>