Implement methods without using "String".

Release Notes

1.5.0

• Using latest typescript version, please check if your environment still works with this version before upgrading!
• feature: !IMPORTANT methods/properties starting with uppercase are marked deprecated now and will be removed in future versions! Check the deprecated infos
• feature: String.isNullOrWhiteSpace accepts null or undefined now
• feature: added new methods/properties: isNullOrWhiteSpace(), formatString(), joinString(), empty, check README
• chore: Updated packages
• chore: added linting