cmake: Add dev-mode CMake preset

To use, invoke `cmake` with argument `--preset dev-mode`.

Solves one item in #1235.

One disadvantage over `./configure --enable-dev-mode` is that CMake
does not provide a way to "hide" presets from users. That is,
`cmake --list-presets` will list dev-mode, and it will also appear
in `cmake-gui`, even though it's not selectable there due to bug
https://gitlab.kitware.com/cmake/cmake/-/issues/23341. (So in our
case, that's probably rather a feature than a bug.)

We curently use version 3 presets which require CMake 3.21+.
Unfortunately, CMake versions before 3.19 may ignore the `--preset`
argument silently. So if the preset is not picked up, make sure you
have a recent enough CMake version.

More unfortunately, we can't even spell this warning out in
CMakePresets.json because CMake does not support officially support
comments in JSON, see
 - https://gitlab.kitware.com/cmake/cmake/-/issues/21858
 - https://gitlab.kitware.com/cmake/cmake/-/merge_requests/5853 .
We could use a hack hinted at in
https://gitlab.kitware.com/cmake/cmake/-/issues/21858#note_908543
but that's risky, because it could simply break for future versions,
and we probably want to use presets not only for dev mode.

CMakePresets.json

@@ -0,0 +1,19 @@
+{
+  "cmakeMinimumRequired": {"major": 3, "minor": 21, "patch": 0}, 
+  "version": 3,
+  "configurePresets": [
+    {
+      "name": "dev-mode",
+      "displayName": "Development mode (intended only for developers of the library)",
+      "cacheVariables": {
+        "SECP256K1_EXPERIMENTAL": "ON",
+        "SECP256K1_ENABLE_MODULE_RECOVERY": "ON",
+        "SECP256K1_BUILD_EXAMPLES": "ON"
+      },
+      "warnings": {
+        "dev": true,
+        "uninitialized": true
+      }
+    }
+  ]
+}