editor: clangd and cmake work from a committed vscode workspace

The three files libavr's consumers carry: .vscode/settings.json names the
atmega328p-generated database and passes --query-driver, .clangd holds the
stand-ins clang needs for GCC's AVR dialect -- and no database, because this
driver is made to be vendored and the file travels with it -- and
extensions.json names the two extensions. The libavr pin advances to the
editor-audit fixes, without which every TU inherits device.hpp's
alias-shadowing errors. The preset builds green from the pin and clangd
reports zero errors on the example TU.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.vscode/extensions.json

@@ -0,0 +1,6 @@
+{
+	"recommendations": [
+		"llvm-vs-code-extensions.vscode-clangd",
+		"ms-vscode.cmake-tools"
+	]
+}