Add decimal parsing support (#11640)

* docs: add DecimalLiteral to AST spec

* add decimal support

* fix: throw invalid decimal on start

* add DecimalLiteral type definitions

* update parser typings

* add generator support

* add syntax-decimal plugin

* Add syntax-decimal to babel-standalone

* add syntax-decimal to missing plugin helpers

* fix incorrect test macro

packages/babel-plugin-syntax-decimal/.npmignore

@@ -0,0 +1,3 @@
+src
+test
+*.log

packages/babel-plugin-syntax-decimal/README.md

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+# @babel/plugin-syntax-decimal
+
+> Allow parsing of decimal
+
+See our website [@babel/plugin-syntax-decimal](https://babeljs.io/docs/en/next/babel-plugin-syntax-decimal.html) for more information.
+
+## Install
+
+Using npm:
+
+```sh
+npm install --save-dev @babel/plugin-syntax-decimal
+```
+
+or using yarn:
+
+```sh
+yarn add @babel/plugin-syntax-decimal --dev
+```

packages/babel-plugin-syntax-decimal/package.json

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+{
+  "name": "@babel/plugin-syntax-decimal",
+  "version": "7.10.1",
+  "description": "Allow parsing of decimal",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/babel/babel.git",
+    "directory": "packages/babel-plugin-syntax-decimal"
+  },
+  "license": "MIT",
+  "publishConfig": {
+    "access": "public"
+  },
+  "main": "lib/index.js",
+  "exports": {
+    ".": "./lib/index.js"
+  },
+  "keywords": [
+    "babel-plugin"
+  ],
+  "dependencies": {
+    "@babel/helper-plugin-utils": "^7.10.1"
+  },
+  "peerDependencies": {
+    "@babel/core": "^7.0.0-0"
+  }
+}

packages/babel-plugin-syntax-decimal/src/index.js

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+import { declare } from "@babel/helper-plugin-utils";
+
+export default declare(api => {
+  api.assertVersion(7);
+
+  return {
+    name: "syntax-decimal",
+
+    manipulateOptions(opts, parserOpts) {
+      parserOpts.plugins.push("decimal");
+    },
+  };
+});