macos: add root-level .swiftlint.yml (#10890)

In order to support running from both the repository root and from
within Xcode project, and to keep things generally organized, our
primary .swiftlint.yml configuration file lives under macos/.

This change introduces a root-level .swiftlint.yml which limits the file
scope to macos/ and then includes macos/.swiftlint.yml for the rest of
the directives.

This unlocks a few benefits:

- We no longer need to pass an explicit `macos` path argument in any of
our invocations. SwiftLint will do the right thing when run either from
the repository root or from within the macos/ directory.
- It lets us easily exclude the macos/build/ directory (and re-enable
the 'deployment_target' rule). In the previous setup, this was more
challenging than you'd expect due to SwiftLint's path resolution rules
and required passing even more arguments like `--working-directory`.

The only downside is adding a new file to the repository root, but that
feels like the right trade-off given the benefits and conveniences.

.github/workflows/test.yml

@@ -23,6 +23,7 @@ jobs:
         with:
           filters: |
             macos:
+              - '.swiftlint.yml'
               - 'macos/**'
 
   required:
@@ -963,7 +964,7 @@ jobs:
           useDaemon: false # sometimes fails on short jobs
 
       - name: swiftlint check
-        run: nix develop -c swiftlint lint --strict macos
+        run: nix develop -c swiftlint lint --strict
 
   alejandra:
     if: github.repository == 'ghostty-org/ghostty'