Add some string functions in built-in functions

[nafasebra]

I've add these strings functions:
char_at
to_lower
to_upper
trim

[ali77gh]

@nafasebra
Be careful in Chap We don't start from 0 in indexes we start at 1

So:

"Nafas",1 -> char_at 

should return 'N' and not "a"

[nafasebra]

@nafasebra Be careful in Chap We don't start from 0 in indexes we start at 1

So:

"Nafas",1 -> char_at 

should return 'N' and not "a"

Ok, I've fixed this

[nafasebra]

Do you approve it?
@ali77gh

[ali77gh]

Congratulations!🎉

Thanks for your contribution