feat(is): add some type guard (#25)

Author: LuciNyan

src/is.ts

@@ -6,6 +6,10 @@ export const isFunction = <T extends Function> (val: any): val is T => typeof va
 exportconstisNumber=(val: any): val is number=>typeofval==='number'
 exportconstisString=(val: unknown): val is string=>typeofval==='string'
 exportconstisObject=(val: any): val is object=>toString(val)==='[object Object]'
+exportconstisUndefined=(val: any): val is undefined=>toString(val)==='[object Undefined]'
+exportconstisNull=(val: any): val is null=>toString(val)==='[object Null]'
+exportconstisRegExp=(val: any): val is RegExp=>toString(val)==='[object RegExp]'
+exportconstisDate=(val: any): val is Date=>toString(val)==='[object Date]'
 
 // @ts-ignore
 exportconstisWindow=(val: any): boolean=>typeofwindow!=='undefined'&&toString(val)==='[object Window]'