Add Rust Analyzer configuration

Add a Rust Analyzer configuration via a VS Code settings file, which
silences some annoying false positive diagnostics.

One can use [folke/neoconf.nvim](https://github.com/folke/neoconf.nvim)
to use the configuration with neovim. I couldn't get this to work with
neoconf.nvim via a .neoconf.json file for some reason.

.vscode/settings.json

@@ -0,0 +1,7 @@
+{
+    "rust-analyzer.checkOnSave.allTargets": false,
+    "rust-analyzer.checkOnSave.extraArgs": [
+        "--target",
+        "thumbv7m-none-eabihf"
+    ]
+}