feat: Add prefixfree (#34164)

* feat: Add `prefixfree`

* chore(prefixfree): Require TypeScript 2.8

* fix(prefixfree): Enable `strictNullChecks`

Co-Authored-By: Nathan Shively-Sanders <293473+sandersn@users.noreply.github.com>

types/prefixfree/.editorconfig

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+# This package uses tabs
+[*]
+indent_style = tab

types/prefixfree/index.d.ts

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+// Type definitions for prefixfree 1.0
+// Project: https://github.com/LeaVerou/prefixfree
+// Definitions by: ExE Boss <https://github.com/ExE-Boss>
+// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// TypeScript Version: 2.8
+
+declare namespace StyleFix {
+	type StyleFixer = (css: string, raw: boolean, element: Element) => string;
+}
+
+interface StyleFix {
+	fix: StyleFix.StyleFixer;
+	fixers?: StyleFix.StyleFixer[];
+	optIn: boolean;
+	camelCase(str: string): string;
+	deCamelCase(str: string): string;
+	link(link: HTMLLinkElement): void;
+	process(): void;
+	register(fixer: StyleFix.StyleFixer, index?: number): void;
+	styleAttribute(element: ElementCSSInlineStyle): void;
+	styleElement(style: HTMLStyleElement): void;
+}
+
+declare const StyleFix: StyleFix;
+
+// PrefixFree
+declare namespace PrefixFree {
+	/** The known prefixes used for CSS properties. */
+	type KnownPrefixCSS = "-moz-" | "-ms-" | "-o-" | "-webkit-";
+
+	/** The known prefixes used for CSS properties in `CSSStyleDeclaration`. */
+	type KnownPrefixJS = "Moz" | "Ms" | "O" | "Webkit";
+}
+
+interface PrefixFree {
+	/**
+	 * The prefix used for CSS properties.
+	 *
+	 * Eg.: `-moz-`, `-ms-`, `-o-` or `-webkit-`.
+	 */
+	prefix: PrefixFree.KnownPrefixCSS | string;
+
+	/**
+	 * The prefix used for CSS properties in `CSSStyleDeclaration`.
+	 *
+	 * Eg.: `Moz`, `Ms`, `O` or `Webkit`.
+	 */
+	Prefix: PrefixFree.KnownPrefixJS | string;
+
+	atrules: string[];
+	functions: string[];
+	keywords: string[];
+	properties: string[];
+	selectorMap: {
+		[selector: string]: string;
+	};
+	selectors: string[];
+	valueProperties: string[];
+
+	prefixCSS: StyleFix.StyleFixer;
+	prefixProperty(property: string, camelCase?: boolean): string;
+	prefixSelector(selector: string): string;
+	property(property: string): string;
+	value(value: string, property?: string): string;
+}
+
+declare const PrefixFree: PrefixFree;
+
+interface Window {
+	StyleFix: StyleFix;
+	PrefixFree: PrefixFree;
+}

types/prefixfree/prefixfree-tests.ts

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+window.StyleFix === StyleFix;
+window.PrefixFree === PrefixFree;
+
+StyleFix.register((css, raw, element) => {
+	return css + "\n/* Custom end comment */\n";
+});
+StyleFix.process();
+
+const browser = (() => {
+	switch (PrefixFree.prefix) {
+		case "-moz-":
+			return "firefox";
+		case "-ms-":
+			return "ie";
+		case "-o-":
+			return "opera";
+		case "-webkit-":
+			return "chrome";
+		default:
+			return "unknown";
+	}
+})();
+
+// $ExpectError
+if (browser === "safari") {
+	// There's no way to differentiate Safari from Chrome using just prefixes
+}

types/prefixfree/tsconfig.json

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+{
+	"compilerOptions": {
+		"module": "commonjs",
+		"lib": [
+			"dom",
+			"es5"
+		],
+		"noImplicitAny": true,
+		"noImplicitThis": true,
+		"strictNullChecks": true,
+		"strictFunctionTypes": true,
+		"baseUrl": "../",
+		"typeRoots": [
+			"../"
+		],
+		"types": [],
+		"noEmit": true,
+		"forceConsistentCasingInFileNames": true
+	},
+	"files": [
+		"index.d.ts",
+		"prefixfree-tests.ts"
+	]
+}

types/prefixfree/tslint.json

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+{
+	"extends": "dtslint/dt.json",
+	"rules": {
+		"indent": [
+			true,
+			"tabs"
+		]
+	}
+}