Improve docs for implementing macros in an existing Swift package

[ianthetechie]

This contribution improves the documentation for getting started with macros in an existing Swift Package. Notably, it is not obvious to first-time users that CompilerPluginSupport needs to be imported, and the default platform support is too low to support macros (in Xcode 15 at the time of this writing).

It also adds imports ahead of the FourCharacterCode macro implementation example, since these will be needed to make any(?) macros compile. Tangentially, it looks like #182 addresses the remaining things that tripped me up initially. These combined additions will smooth out the intro to macros significantly.

[amartini51]

Thanks for these additions! I took a pass at rewriting some of the prose.

[ianthetechie]

Thanks for the review @amartini51! I've pushed a few changes.

1. Accept your suggestion re: the bulleted list. Good call.
2. I noticed that the use of ending punctuation was inconsistent. I added it, as that seemed to be the norm, but let me know if you'd prefer otherwise.
3. I reorganized the part about adding the SwiftSyntax dependency to be in the list + Package.swift sample. That left the remaining original text a bit sparse (another single-sentence paragraph). I'd welcome any style suggestions on that.

[ianthetechie]

Quick bump on this as it's getting a bit stale and now merge conflicts are starting to pop up.

[amartini51]

@ianthetechie I'll take another look at this — I started a rewrite pass, but then had to focus on getting TSPL updated for Swift 5.9.2 beta 3. Thanks for resolving the merge conflict. I was expecting to see some conflicts, since other PRs also modified some of the same content as this one.

There's a balance in this PR — which I also had to make in the first version — between providing enough information about using Swift Syntax for folks to get started, and not trying to actually teach too much of Swift Syntax here. I'm also trying to make some adjustments to keep the order of steps linear, with alternate instructions at some points for new/existing code, rather than folding too many steps together under an "existing project" alternative.

A general workflow comment: force-pushing to a branch during PR review makes that review more difficult. As a reviewer, I can no longer easily see what changed since the last time I looked at the PR. I'm aware that workflows vary here, so you may have gotten the opposite guidance from others in the past. (For more details, please see CONTRIBUTING in this repo.)

[ianthetechie]

Understood; sorry about that force push!

[amartini51]

@ianthetechie How's my latest commit look to you? Some content that was recently merged into 'main' got deleted as part of merging that branch in here, which I restored. I also removed some duplicate discussion of SwiftSyntax. My intended approach was to keep the same step-by-step ordering in the content, but to add "if new" and "if existing" versions of the steps where needed.

[ianthetechie]

Thanks @amartini51! I think this revision is much better, and I appreciate your attention to the flow of things. All looks good to me.