fix: check path is string in pathToFsPath

There's a chance this function can be called with a path that is not a string.

To catch that, we check if path is of a different type and throw an error if it
is.

This also adds a couple tests for this function.

test/unit/node/util.test.ts

@@ -453,3 +453,44 @@ describe("escapeHtml", () => {
     )
   })
 })
+
+describe("pathToFsPath", () => {
+  it("should convert a path to a file system path", () => {
+    expect(util.pathToFsPath("/foo/bar/baz")).toBe("/foo/bar/baz")
+  })
+  it("should lowercase drive letter casing by default", () => {
+    expect(util.pathToFsPath("/C:/far/boo")).toBe("c:/far/boo")
+  })
+  it("should keep drive letter casing when set to true", () => {
+    expect(util.pathToFsPath("/C:/far/bo", true)).toBe("C:/far/bo")
+  })
+  it("should throw an error if a non-string is passed in for path", () => {
+    expect(() =>
+      util
+        // @ts-expect-error We need to check other types
+        .pathToFsPath({}),
+    ).toThrow(
+      `Could not compute fsPath from given uri. Expected path to be of type string, but was of type undefined.`,
+    )
+  })
+  it("should not throw an error for a string array", () => {
+    // @ts-expect-error We need to check other types
+    expect(() => util.pathToFsPath(["/hello/foo", "/hello/bar"]).not.toThrow())
+  })
+  it("should use the first string in a string array", () => {
+    expect(util.pathToFsPath(["/hello/foo", "/hello/bar"])).toBe("/hello/foo")
+  })
+  it("should replace / with \\ on Windows", () => {
+    let ORIGINAL_PLATFORM = process.platform
+
+    Object.defineProperty(process, "platform", {
+      value: "win32",
+    })
+
+    expect(util.pathToFsPath("/C:/far/boo")).toBe("c:\\far\\boo")
+
+    Object.defineProperty(process, "platform", {
+      value: ORIGINAL_PLATFORM,
+    })
+  })
+})