Improve project importing and CMake Tools integration for pico-examples

[will-v-pi]

The pico-examples repository is just one example, but there are a couple of improvements that I think will help with importing other projects too:

• Add/replace set(PICO_BOARD xxx CACHE STRING "Board type") to the imported project CMakeLists.txt - and also add board selection to the Import Project webview
• Detect whether a project will require CMake Tools, and switch to that (maybe with a popup to ask the user) - this will be for any project where there are multiple executables (eg pico-examples)
• As part of this, maybe add a command to toggle between CMake Tools and this extension for compiling - this would toggle
  • "raspberry-pi-pico.cmakeAutoConfigure": true -> false
  • "raspberry-pi-pico.useCmakeTools": false -> true
  • "cmake.configureOnEdit": false -> true
  • "cmake.automaticReconfigure": false -> true
  • "cmake.configureOnOpen": false -> true
• Use the "Configure CMake" button to trigger configuring the CMake Tools extension (like we do for the launchTargetPath stuff)
• Remove "toolchainFile" from generated cmake-kits.json - it's not necessary, as the SDK configures the correct one