feat: adds TypeScript definitions

fixes bcoe#195

index.d.ts

@@ -0,0 +1,23 @@
+export type Watermark = [number, number];
+
+export declare class Report {
+    constructor(opts: {
+        exclude?: any[],
+        include?: any[],
+        reporter: string[],
+        reportsDirectory?: string,
+        tempDirectory?: string,
+        watermarks?: Partial<{
+            statements: Watermark;
+            functions: Watermark;
+            branches: Watermark;
+            lines: Watermark;
+        }>,
+        omitRelative?: boolean,
+        wrapperLength?: number,
+        resolve?: string,
+        all?: boolean,
+    })
+
+    run(): Promise<void>;
+}

index.js

@@ -0,0 +1 @@
+exports.Report = require('./lib/report')

package.json

@@ -2,7 +2,8 @@
   "name": "c8",
   "version": "7.0.1",
   "description": "output coverage reports using Node.js' built in coverage",
-  "main": "index.js",
+  "main": "./index.js",
+  "types": "./index.d.ts",
   "bin": "./bin/c8.js",
   "repository": {
     "type": "git",
@@ -58,6 +59,8 @@
     "node": ">=10.12.0"
   },
   "files": [
+    "index.js",
+    "index.d.ts",
     "lib",
     "bin",
     "LICENSE"