Add TypeScript definition (#13)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

index.d.ts

@@ -0,0 +1,19 @@
+export interface EnvironmentVariables {
+    readonly [key: string]: string;
+}
+
+/**
+ * Get the environment variables defined in your dotfiles.
+ *
+ * @param shell - The shell to read the environment variables from. Default: User default shell.
+ * @returns The environment variables.
+ */
+export function sync(shell?: string): EnvironmentVariables;
+
+/**
+ * Get the environment variables defined in your dotfiles.
+ * 
+ * @param shell - The shell to read the environment variables from. Default: User default shell.
+ * @returns The environment variables.
+ */
+export default function shellEnv(shell?: string): Promise<EnvironmentVariables>;

index.test-d.ts

@@ -0,0 +1,5 @@
+import {expectType} from 'tsd-check';
+import shellEnv, {sync, EnvironmentVariables} from '.';
+
+expectType<EnvironmentVariables>(sync());
+expectType<EnvironmentVariables>(await shellEnv());

package.json

@@ -13,10 +13,11 @@
 		"node": ">=6"
 	},
 	"scripts": {
-		"test": "xo && ava"
+		"test": "xo && ava && tsd-check"
 	},
 	"files": [
-		"index.js"
+		"index.js",
+		"index.d.ts"
 	],
 	"keywords": [
 		"env",
@@ -39,6 +40,7 @@
 	},
 	"devDependencies": {
 		"ava": "*",
+		"tsd-check": "^0.3.0",
 		"xo": "*"
 	}
 }