WIP: gulp 4.x compat

AssertionError [ERR_ASSERTION]: Task function must be specified
AssertionError [ERR_ASSERTION]: Task never defined: readme
  gulpjs/gulp#802
[10:31:47] The following tasks did not complete: check-priv
[10:31:47] Did you forget to signal async completion?
[10:53:00] Error: watching conf.priv.js,completions.js,conf.js,actions.js,help.js,keys.js,util.js,gulpfile.js: watch task has to be a function (optionally generated by using gulp.parallel or gulp.series)

gulpfile.js

@@ -57,32 +57,16 @@ gulp.task("lint", () =>
     .pipe(eslint())
     .pipe(eslint.format()))
 
-gulp.task("check-priv", () => {
+gulp.task("check-priv", (done) => {
   try {
     fs.statSync("./conf.priv.js")
   } catch (e) {
     // eslint-disable-next-line no-console
     console.log("Creating ./conf.priv.js based on ./conf.priv.example.js")
     fs.copyFileSync("./conf.priv.example.js", "./conf.priv.js", fs.constants.COPYFILE_EXCL)
   }
-})
-
-gulp.task("build", ["check-priv", "clean", "lint", "readme"], () => gulp.src(paths.entry, { read: false })
-  .pipe(parcel())
-  .pipe(rename(".surfingkeys"))
-  .pipe(gulp.dest("build")))
-
-gulp.task("install", ["build"], () => gulp.src("build/.surfingkeys")
-  .pipe(gulp.dest(os.homedir())))
-
-gulp.task("watch", () => {
-  gulp.watch([].concat(paths.scripts, paths.gulpfile), ["readme", "install"])
-  gulp.watch(paths.readme, ["readme"])
-})
 
-gulp.task("watch-nogulpfile", () => {
-  gulp.watch([].concat(paths.scripts), ["readme", "install"])
-  gulp.watch(paths.readme, ["readme"])
+  done()
 })
 
 gulp.task("readme", () => {
@@ -154,4 +138,26 @@ gulp.task("readme", () => {
     .pipe(gulp.dest("."))
 })
 
-gulp.task("default", ["build"])
+gulp.task("build", gulp.series("check-priv", "clean", "lint", "readme"), (done) => {
+  gulp.src(paths.entry, { read: false })
+    .pipe(parcel())
+    .pipe(rename(".surfingkeys"))
+    .pipe(gulp.dest("build"))
+
+  done()
+})
+
+gulp.task("install", gulp.series("build"), () => gulp.src("build/.surfingkeys")
+  .pipe(gulp.dest(os.homedir())))
+
+gulp.task("watch", () => {
+  gulp.watch([].concat(paths.scripts, paths.gulpfile), gulp.series("readme", "install"))
+  gulp.watch(paths.readme, gulp.series("readme"))
+})
+
+gulp.task("watch-nogulpfile", () => {
+  gulp.watch([].concat(paths.scripts), gulp.series("readme", "install"))
+  gulp.watch(paths.readme, gulp.series("readme"))
+})
+
+gulp.task("default", gulp.series("build"))