[Feature] Add cpad function to stdlib.text

[ajwalkiewicz]

Problem
Centering a text requires writing own custom function, I realized than when I wanted to create a nicely looking separators for the console output of my script, for example:

======== Stage 1 ========
# scrip does some action
======== Stage 2 ========
# another, etc.

Proposal
I propose a simple cpad function that is build on top of lpad and rpad:

/// Pads `text` with the specified `pad` character to the center within the desired `length`
///
/// ### Usage
/// ```ab
/// let padded: Text
///
/// padded = cpad("42", "0", 5)
/// echo padded // "04200"
///
/// padded = cpad("42", "0", 6)
/// echo padded // "004200"
///
/// padded = cpad("42", "0", 1)
/// echo padded // "42"
/// ```
pub fun cpad(text: Text, pad: Text, length: Int): Text {
    const text_length = len(text)
    if length <= text_length: return text

    const left_padding_length = length / 2 + text_length / 2
    const left_padded = lpad(text, pad, left_padding_length)
    const center_padded = rpad(left_padded, pad, length)

    return center_padded
}

Alternatives

1. Alternative is to not have it. It is rather nice to have feature not must have. Some languages don't have such feature, e.g. Java, though they are popular libraries that allows that. On the other hand some popular scripting languages like Python or Ruby have it, so I would opt for having it too.
2. To not build it on top of lpad and rpad, but that I'm afraid would duplicate the logic from those functions anyway.

[ajwalkiewicz]

I see there are many other more important things to do in the project, unfortunately I'm not good at rust, so can't help with them. Anyway, let me know if this is something that you even want, and I can create the PR.

[Mte90]

Contribute to the stdlib doest' require rust but to write the function with tests in pure amber.