Add a package for localizing monetary amounts

This library was written for the in-progress composite-checkout package but has no external dependencies, so we're extracting it as its own package. Features: exact arithmetic and non-breaking spaces

packages/localize-monetary-amount/README.md

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+# localize-monetary-amount
+
+Locale- (language and geo) and currency-aware formatting of exact monetary amounts.
+
+What this package does:
+  * Number localization (separator symbols, grouping conventions)
+  * Integer arithmetic with all digits of precision. No floats.
+  * Designed for displaying product prices in catalogs, carts, checkout, and receipts.
+  * No dependencies
+
+What this package doesn't do:
+  * Arithmetic on monetary amounts; formatting only
+  * Conversion between currencies
+  * Localize digit symbols (Hindu-Arabic digits only)
+  * Take options. Locale preferences are hardcoded. Please submit an issue if a locale looks off.
+
+## Example usage
+
+You'll need:
+  * An ISO 631-1 language code with an optional ISO 3166-1 alpha-2 region code, separated by a hyphen. Examples: `en`, `en-gb`, `fr-be`.
+  * An ISO 4217 currency code. Examples: 'USD', 'JPY', 'BRL'.
+  * An integer number of _minor units_. This is the minimal unit of your currency; e.g. cents for USD, yen for JPY. May be positive or negative.
+
+If the fractional part is zero it is omitted unless the locale prefers otherwise.
+
+```js
+import localizeMonetaryAmount from 'localize-monetary-amount';
+
+// Strips zero minor units:
+localizeMonetaryAmount( 'en-us', 'USD', 500 ); // '$5'
+
+// Uses non-breaking spaces:
+localizeMonetaryAmount( 'fr-ca', 'CAD', 500000 ); // '$5\u00A0000\u00A0CAD'
+```

packages/localize-monetary-amount/jest.config.js

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+module.exports = { rootDir: __dirname, testMatch: [ '**/test/**/*.[jt]s?(x)' ] };

packages/localize-monetary-amount/package.json

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+{
+	"name": "@automattic/localize-monetary-amount",
+	"version": "1.0.0",
+	"description": "Batteries included localized currency formatting",
+	"main": "dist/cjs/index.js",
+	"module": "dist/esm/index.js",
+	"sideEffects": false,
+	"scripts": {
+		"clean": "npx rimraf dist",
+		"prepublish": "npm run clean"
+	},
+	"files": [
+		"dist",
+		"src"
+	],
+	"keywords": [
+		"localization",
+		"currency",
+		"automattic"
+	],
+	"publishConfig": {
+		"access": "public"
+	},
+	"repository": {
+		"type": "git",
+		"url": "git+https://github.com/Automattic/wp-calypso.git",
+		"directory": "packages/localize-monetary-amount"
+	},
+	"author": "@automattic",
+	"license": "GPL-2.0-or-later",
+	"bugs": {
+		"url": "https://github.com/Automattic/wp-calypso/issues"
+	},
+	"homepage": "https://github.com/Automattic/wp-calypso/tree/master/packages/localized-monetary-amount#readme"
+}