Add string utils

[drebrez]

Would be nice to have the following string utility functions:

• isNullOrEmpty
• isNullOrWhiteSpace
• capitalize

In my opinion they should behave like the c# implementation (also tabs/newlines/carriagereturns/... are considered whitespaces)

Here some possible implementations => https://stackoverflow.com/a/32800728

Maybe even provide a StringBuilder class?

[drebrez]

some other interesting utilities => https://github.com/microsoft/vscode/blob/main/src/vs/base/common/strings.ts