Added the following stocks:
- IsStringNumber
- IsStringFloat
- StringToLowerCase
- StringToUpperCase
- IndexOfChar
- LastIndexOfChar

Tested:
Test plugin source link: https://pastebin.com/QWXUy0ta
Test results: https://i.imgur.com/NKhvsl4.png

Removed unnecessary temporary variables.

Changed IsStringNumber and IsStringFloat to wrap StringToIntEx and StringToFloatEx.
Changed IndexOfChar to wrap FindCharInString.
Added CopyC

If the string has a floating point value or not, we can still store it in a float.
No need to check the additional stocks, they're built-in.
CopyC already exists, that's simply strcopy.