fix: allow format calls with one side of (file + ext) missing

In node, the following are valid calls of `format`:

```ts
format({ file: 'foo' });
// foo

format({ dir: '/foo', name: 'bar' });
// /foo/bar

format({ dir: '/foo', ext: 'bar' });
// /foo/.bar
```

The last one is a little nonsensical, but the other two are valid
(extensionless files for example). We should also be matching node's
behaviour of course.

Also enables `strict: true` in TypeScript, which is what caught this.

src/_path.ts

@@ -250,7 +250,10 @@ export const dirname: typeof path.dirname = function (p) {
 };
 
 export const format: typeof path.format = function (p) {
-  const segments = [p.root, p.dir, p.base ?? p.name + p.ext].filter(Boolean);
+  const ext = p.ext ? (p.ext.startsWith(".") ? p.ext : `.${p.ext}`) : "";
+  const segments = [p.root, p.dir, p.base ?? (p.name ?? "") + ext].filter(
+    Boolean,
+  ) as string[];
   return normalizeWindowsPath(
     p.root ? resolve(...segments) : segments.join("/"),
   );

test/index.spec.ts

@@ -153,6 +153,10 @@ runTest("format", format, [
   [{ root: "/", base: "file.txt", ext: "ignored" }, "/file.txt"],
   [{ root: "/", name: "file", ext: ".txt" }, "/file.txt"],
   [{ name: "file", ext: ".txt" }, "file.txt"],
+  [{ name: "file" }, "file"],
+  [{ ext: "ext" }, ".ext"],
+  [{ ext: ".ext" }, ".ext"],
+  [{ dir: "/foo", name: "file" }, "/foo/file"],
 
   // Windows
   [{ name: "file", base: "file.txt" }, "file.txt"],