Introduce Swiftlint (#572)

Trying out a basic set of rules for catching mistakes like line length
etc. everything is configured as a warning rather than an error right
now so the build will never fail for lint reasons. I started with all
the rules on and removed anything that was obviously in conflict with
our existing style / rules.

We have ~500 lint warnings in the codebase atm and it would be good to
fix them because github actions can only display a maximum of 10 linting
errors per run (yeah, I know). So lint errors introduced in a PR won't
actually show up in the UI until we clear the backlog of underlying lint
issues.

The github action I used (made by the author of Swiftlint) purports to
be able to scope the linting to only files changed in the PR but I get
errors whenever I try to configure it as per their example.

---

Swiftlint can fix basic issues by running `swiftlint --fix` but we need
to pick our moment carefully to avoid conflicts with in-flight work.

You can also choose to integrate swiftlint into Xcode as a build action
(see this draft PR
#573). If we
gain enough confidence in rules I would be tempted to add a pre-commit
hook that autofixes.

.github/workflows/swiftlint.yaml

@@ -0,0 +1,16 @@
+on:
+  pull_request:
+    paths:
+      - '.github/workflows/swiftlint.yml'
+      - '.swiftlint.yml'
+      - '**/*.swift'
+
+name: SwiftLint
+
+jobs:
+  SwiftLint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: GitHub Action for SwiftLint
+        uses: norio-nomura/action-swiftlint@master

.swiftlint.yml

@@ -0,0 +1,19 @@
+only_rules:
+  - line_length
+  - vertical_whitespace
+  - redundant_discardable_let
+  - unused_closure_parameter
+  - mark
+  - implicit_getter
+  - trailing_comma
+  - orphaned_doc_comment
+  - control_statement
+  - multiple_closures_with_trailing_closure
+  - opening_brace
+  - empty_enum_arguments
+  - for_where
+  - redundant_string_enum_value
+  - colon
+  - syntactic_sugar
+
+line_length: 100