chore: migrate test cases from Chai to Jest

src/fs/fs-impl.ts

@@ -0,0 +1,49 @@
+import { promisify } from '../util'
+import { sep, resolve as nodeResolve, extname, dirname as nodeDirname } from 'path'
+import { stat, statSync, readFile as nodeReadFile, readFileSync as nodeReadFileSync } from 'fs'
+import { requireResolve } from './node-require'
+
+const statAsync = promisify(stat)
+const readFileAsync = promisify<string, string, string>(nodeReadFile)
+
+export async function exists (filepath: string) {
+  try {
+    await statAsync(filepath)
+    return true
+  } catch (err) {
+    return false
+  }
+}
+export function readFile (filepath: string) {
+  return readFileAsync(filepath, 'utf8')
+}
+export function existsSync (filepath: string) {
+  try {
+    statSync(filepath)
+    return true
+  } catch (err) {
+    return false
+  }
+}
+export function readFileSync (filepath: string) {
+  return nodeReadFileSync(filepath, 'utf8')
+}
+export function resolve (root: string, file: string, ext: string) {
+  if (!extname(file)) file += ext
+  return nodeResolve(root, file)
+}
+export function fallback (file: string) {
+  try {
+    return requireResolve(file)
+  } catch (e) {}
+}
+export function dirname (filepath: string) {
+  return nodeDirname(filepath)
+}
+export function contains (root: string, file: string) {
+  root = nodeResolve(root)
+  root = root.endsWith(sep) ? root : root + sep
+  return file.startsWith(root)
+}
+
+export { sep } from 'path'