Generate compile_commands.json

Modify CMakePresets.json to use -DCMAKE_EXPORT_COMPILE_COMMANDS:BOOL=ON
in all the presets.

The compile_commands.json file can be used by tools like clangd to get
autocompletion and go-to-definition.

This patch also adds compile_commands.json to the .gitignore, as often
you need to symlink the compile_commands.json from the build directory
into the source directory for it to be picked by tools. That is the case
with clangd at least.

CMakePresets.json

@@ -10,6 +10,10 @@
         "name": "common",
         "hidden": true,
         "cacheVariables": {
+          "CMAKE_EXPORT_COMPILE_COMMANDS": {
+            "type": "BOOL",
+            "value": "ON"
+          }
         }
       },
       {