Add gofumpt-tool.sh script and use it in VS Code

.vscode/settings.json

@@ -1,6 +1,6 @@
 {
   "gopls": {
-    "formatting.gofumpt": true,
+    "formatting.gofumpt": false,
     "ui.diagnostic.staticcheck": true,
     "ui.diagnostic.analyses": {
       // This list must match the one in .golangci.yml
@@ -24,6 +24,8 @@
   },
   "go.alternateTools": {
     "golangci-lint-v2": "${workspaceFolder}/scripts/golangci-lint-shim.sh",
+    "customFormatter": "${workspaceFolder}/scripts/gofumpt-tool.sh",
   },
   "go.lintTool": "golangci-lint-v2",
+  "go.formatTool": "custom",
 }

scripts/gofumpt-tool.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+# This is used by VSCode; it is not very useful otherwise, since it's easy
+# enough to just run `go tool gofumpt` directly, or use `just format`.
+
+set -e
+
+script_dir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+repo_root=$(dirname "$script_dir")
+
+cd "$repo_root"
+exec go tool gofumpt "$@"