ci: add golangci-lint and fix existing lint failures (#118)

This PR adds `golangci-lint` based on the configuration from
`coder/coder`
([here](https://github.com/coder/coder/blob/main/.golangci.yaml)) then
migrated to v2 using `golangci-lint migrate` plus the addition of few
more linters.

---------

Signed-off-by: Callum Styan <callumstyan@gmail.com>

.github/workflows/golangci-lint.yml

@@ -0,0 +1,24 @@
+name: golangci-lint
+on:
+  push:
+    branches:
+      - main
+      - master
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  golangci:
+    name: lint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
+        with:
+          go-version: stable
+      - name: golangci-lint
+        uses: golangci/golangci-lint-action@v8
+        with:
+          version: v2.1