The main benefit this brings is it brings more alignment with the
`clause` arguments for `expect` calls. The docs will be able to focus on
the difference in how the value is use (preceding "that" in the case of
labels, standing on its own in a list in the case of clauses) and can
use a consistent description for how it is passed.

A secondary benefit is that it allows multiline labels and avoid
workaround like joining with `r'\n'`.

A final benefit is that it saves some unnecessary String formatting
since the callback isn't called if no expectations fail on the Subject,
or when used as a soft check where the failure details are ignored.

- Make the `label` arguments to `nest` and `nestAsync`, and the _label
  field in `_TestContext` an `Iterable<String> Function()`.
- Wrap strings that had been passed to `String` arguments with callbacks
  that return the string in a list.
- When writing the label in a failure, write all lines, and use a
  postfix " that:".
- Update some `Map` expectations which had manually joined with literal
  slash-n to keep the label or clause to a single line to take advantage
  of the multiline allowance. Split tests for the changed
  implementations and add tests for the descriptions with multiline
  examples. Some of these could have used multiline clauses before.

This aligns these arguments with all the other failure formatting
arguments like `clause` and `label`. There may be some performance
benefit in some cases where a rejection from `softCheck` is ignored and
expensive String operations are avoid, but in the typical case this just
introduces closures which will be invoked shortly.

Replace the default empty list for `actual` with a default function.
This is a slight behavior change where passing `() => []` will not get
overwritten with the default, but passing `[]` would have. Only
defaulting for when the argument was not passed at all is slightly
better behavior.

Replace a bunch of `Iterable<String>` with `Iterable<String> Function()`
and invoke them at the moment the strings are needed.