When option groups are nested, any titles of the nested groups are
removed, even when the containing group doesn't have a title. This
change preserves the nested groups' titles when grouped in an
untitled option group. Nesting within a titled option group continues
to override any nested option groups.

Fixes #585.

Renames an internal property of argument parser to simplify tracking
values through the code base. Should have no functional changes.

Updates string diff printer to fix an OOB indexing bug.

Bash completion scripts: Respects the `extensions` array for `CompletionKind.file()`
and uses `_filedir` when available for `.file` and `.directory`.

Updates HelpGenerator to print possible value options as a suffix to the
user defined help string. In practice this looks like:

> Set diagnostic level to report public declarations without an
> availability attribute. (values: error, warn, ignore; default: warn)

* Respect the `COLUMNS` and `LINES` environment variables, if set, when determining screen size.
* Add test for COLUMNS environment override
* Make columns test idempotent against there being a COLUMNS value already set in the environment
* Make help tests be more explicit about screen widths.

* See issue #561

This change adds conditional `Sendable` conformance to all
property wrapper types when their `Value` is `Sendable`, enabling
commands to be used in concurrent contexts. Some notes on
the implementation:

* Fix flag exclusivity issues

This derives the `hasUpdated` check from the parsed values data type,
rather than storing it in the closure (which breaks sendability) or
passing it through the closure invocation (which wasn't finished
enough to actually work).

* Mark all `transform` methods as `@Sendable`

This allows for a stronger, compiler-supported guarantee of
sendability when a compound `ParsableArguments` or `ParsableCommand`
type is marked `Sendable`. Most transformations shouldn't be a
problem, since the general case is that these are pure string ->
value transformations.

In cases where making such a transformation sendable is impossible,
an author can always change the property to be just a string and
perform the transformation within the context of the command's
execution, in either the `run()` or `validate()` methods.

* Add `@preconcurrency` to Sendable closure APIs

This adds the `@preconcurrency` attribute to all public APIs that
have changed to take a `@Sendable` closure. This will ease the
migration path for sendable adoption for ArgumentParser users, since
a warning will only appear for using these APIs (like the `transform`
parameter in an @option or @argument) once they've turned on strict
concurrency checking.

I'm also backing out changes that avoided those warnings in the tests
and examples, since in most cases those warnings are spurious;
unapplied functions don't capture state. See
https://forums.swift.org/t/pitch-inferring-sendable-for-methods-and-key-path-literals/68011
for more on this and hopefully an upcoming fix for these issues.

* Raise minimum Swift version to 5.7

In order to provide `@preconcurrency` support, the package needs to
have a minimum Swift requirement of 5.7. This makes that change and
updates the README to indicate this for the next version.

Co-authored-by: Kenny York <kenny_york@apple.com>

Update some documentation, mark some code samples as being
Swift so that syntax highlighting works properly, and hide some
other infrequently-used symbols by adding internal parameter
name underscores.

These warnings are showing up in tests about types and protocols
that are defined within this package, so they don't pose a problem
for future library evolution. Instead of using the `@retroactive`
attribute, which isn't supported by older compilers, this change
fully qualifies the type and protocol in the relevant conformance
declarations, which suppresses the issue.

Re: https://github.com/apple/swift-evolution/blob/main/proposals/0364-retroactive-conformance-warning.md

These use APIs which are not available on Windows, so skipping the test
is insufficient as it does not build.