Adding Text.at and Text.is_digit functions

[jdunkerley]

Pull Request Description

Adding Text.at index based access to extended grapheme clusters.
Adding Text.is_digit method.

Important Notes

When using a negative index, the function has to realise the full grapheme cluster set.

Checklist

Please include the following checklist in your PR:

• The documentation has been updated if necessary.
• All code conforms to the Scala, Java, and Rust style guides.
• All documentation and configuration conforms to the markdown and YAML style guides.
• All code has been tested where possible.

[wdanilo]

As always, I love the detailed tests for these functions!