Add a `Text` substring function in codeworld-base

It would be great to add a function to get a subrange of a `Text` value in `codeworld-base`. Something like:

subtext :: (Text, Number, Number) -> Text
subtext (t, a, b) = T.take (b - a) (T.drop a t)

but with appropriate validation for non-integer parameters, etc.

Start by locating the Text API in codeworld-base and reviewing how its existing functions handle numeric arguments; the issue points to T.take and T.drop as the relevant operations. Clarify and implement the requested subtext behavior, including validation for non-integer parameters, then verify the resulting Text ranges and invalid-input cases.

Written by the indexing model from the issue text.