Improve docs for implementing macros in an existing Swift package (#184)

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 macros
compile. These combined additions will smooth out the intro to macros
significantly.

Author: amartini51

TSPL.docc/LanguageGuide/Macros.md

@@ -463,10 +463,32 @@ this creates several files,
 including a template for a macro implementation and declaration.
 
 To add macros to an existing project,
-add a target for the macro implementation
-and a target for the macro library.
+edit the beginning of your `Package.swift` file as follows:
+
+- Set a Swift tools version of 5.9 or later in the `swift-tools-version` comment.
+- Import the `CompilerPluginSupport` module.
+- Include macOS 10.15 as a minimum deployment target in the `platforms` list.
+
+The code below shows the beginning of an example `Package.swift` file.
+
+```swift
+// swift-tools-version: 5.9
+
+import PackageDescription
+import CompilerPluginSupport
+
+let package = Package(
+    name: "MyPackage",
+    platforms: [ .iOS(.v17), .macOS(.v13)],
+    // ...
+)
+```
+
+Next, add a target for the macro implementation
+and a target for the macro library
+to your existing `Package.swift` file.
 For example,
-you can add something like the following to your `Package.swift` file,
+you can add something like the following,
 changing the names to match your project:
 
 ```swift
@@ -514,6 +536,9 @@ consider `#fourCharacterCode` from the previous section.
 Here's a structure that implements that macro:
 
 ```swift
+import SwiftSyntax
+import SwiftSyntaxMacros
+
 public struct FourCharacterCode: ExpressionMacro {
     public static func expansion(
         of node: some FreestandingMacroExpansionSyntax,