Current code formatting in the themes snippet requires horizontal scrolling. If it's slightly reformatted, horizontal scrolling can be avoided improving readability.

Bump version to 0.7.0

…ohnSundell#65)

- Publish now uses the Swift 5.4 toolchain.
- Since Swift 5.4 now enables automatic test discovery on all supported platforms, all of the
   infrastructure that was previously used to maintain Linux unit testing compatibility has now
   been removed.
- Bump Ink to version 0.5.1, which includes a fix for Swift 5.4 compatibility.

- Bump Plot to version `0.9.0`.
- Publish now ships with a few implementations of Plot's new `Component` protocol - specifically
   `Markdown` (for rendering Markdown inline within a component hierarchy), `VideoPlayer` (for
   rendering an inline video player), and an extension that makes it possible to directly use Plot's
  `AudioPlayer` component with Publish's `Audio` model.
- The `Content.Body` type now also conforms to `Component`, which makes it possible to place
   such instances directly within a component hierarchy. That type has now also been fully documented,
   since it was previously missing documentation for some of its properties and initializers.
- The built-in Foundation theme as been rewritten using the new component API. While it remains functionally
   identical to the previous implementation, it should act as a nice example of how this new API can be used.
- Because Publish now ships with a type called `Markdown`, it's possible that some API users might need
   to disambiguate between this new type and Ink's `Markdown` type. However, that tradeoff was considered
   worth it, since using the new `Markdown` component will likely be a much more common use case.

* Checkout target deployment destination
* Init git repository with intended branch
* Create branch if it doesn't exist
* Add test master branch switch

Make the Publish CLI use version 0.8.0 of Publish.

This change adds initial support for running certain publishing operations concurrently
using Swift's new built-in concurrency system. This doesn't involve any breaking changes
to the public API, apart from now requiring all Publish users to ensure that their HTML theme
and multi-item mutation code can be executed in a concurrent manner.

The following key changes were made as part of this:

- Bump the project's Swift version to `5.5`, and update the CLI's project template accordingly.
- Add CollectionConcurrencyKit as a new package dependency, which adds concurrent
   versions of the various sequence mapping functions that Publish makes heavy use of.
- Make `PublishingStep` use an `async` closure type as its body, which in turn enables
   asynchronous calls to be made within any step, both built-in and custom ones.
- Each publishing pipeline is still performed in sequence, just like before. So it's only the
   internal operations within each step that may now be parallelized.
- The top-level `Website.publish` APIs are still completely synchronous, and uses an internal
   `DispatchSemaphore` to await the completion of the website's publishing pipeline before
   returning.
- A new built-in `mutateAllItems(in sections:)` API has been added, to make it convenient
   to iterate over all items within a certain set of sections in a concurrent manner.
- `HTMLGenerator` now executes concurrently, meaning that it can output a large number
   of HTML files in parallel, rather than writing all files one-by-one.
- `MarkdownFileHandler` now also runs concurrently, and processes all of its Markdown files
   in parallel, rather than doing so sequentially.
- `RSSFeedGenerator` and `PodcastFeedGenerator` now generates their respective feeds
   concurrently.

Overall, this set of changes results in a 4x speed improvement when generating Swift by Sundell
using the M1 Max chip.

As part of this, Publish now requires macOS 12, to be able to use Swift Concurrency in a
predictable way. macOS version 12.0 has been out for quite a while now, so it's safe to require
it at this point.

Since Publish now requires macOS 12, we can safely require Python 3 when using
the CLI's `run` command to start a local web server.

…ly maintained