Use module partitions instead of full modules (#1146)

Follow-up to #1015. This pull request replaces the full modules that represent headers, with partitions, to emphasise the belonging of the header to the module. This should hopefully provide a speedup to compilation, and confuse users less by aggregating the usable modules into a smaller set.

doc/cpp20-modules.md

@@ -4,15 +4,15 @@
 > [!WARNING]
 > This feature is still in development, and the API may change in future releases.
 > Your contribution is needed to help us improve the compatibility and usability
-> of C++20 modules in FTXUI. If you encounter any issues or have suggestions,
+> of C++ modules in FTXUI. If you encounter any issues or have suggestions,
 > please open an issue.
 
 FTXUI experimentally supports
 [C++20 modules](https://en.cppreference.com/w/cpp/language/modules) to reduce
-compilation times and improve code organization. Each header has a
-corresponding module.
+compilation times and improve code organization. Each part of the library has a
+corresponding module, split into partitions per each header.
 
-Use the FTXUI_BUILD_MODULES option to build the FTXUI project itself to provide C++ 20 modules,
+Use the FTXUI_BUILD_MODULES option to build the FTXUI project itself to provide C++20 modules,
 for example with CMake and Ninja:
 
 ```sh
@@ -25,7 +25,7 @@ ninja
 ```
 
 > [!NOTE]
-> To use modules, you need a C++20 compatible compiler, CMake version 3.20 or
+> To use modules, you need a C++≥20 compatible compiler, CMake version 3.20 or
 > higher, and use a compatible generator like Ninja. Note that Makefile
 > generators **do not support modules**.
 
@@ -34,9 +34,12 @@ Then, in your own code you can consume the modules and code as normal:
 ```cpp
 import ftxui;
 
+using ftxui::Button;
+using ftxui::ScreenInteractive;
+
 int main() {
-  auto screen = ftxui::ScreenInteractive::TerminalOutput();
-  auto button = ftxui::Button("Click me", screen.QuitClosure());
+  auto screen = ScreenInteractive::TerminalOutput();
+  auto button = Button("Click me", screen.QuitClosure());
   screen.Loop(button);
   return 0;
 }
@@ -70,38 +73,6 @@ are available:
 
 - `ftxui`
     - `ftxui.component`
-      - `ftxui.component.Animation`
-      - `ftxui.component.CapturedMouse`
-      - `ftxui.component.Component`
-      - `ftxui.component.ComponentBase`
-      - `ftxui.component.ComponentOptions`
-      - `ftxui.component.Event`
-      - `ftxui.component.Loop`
-      - `ftxui.component.Mouse`
-      - `ftxui.component.Receiver`
-      - `ftxui.component.ScreenInteractive`
-      - `ftxui.component.Task`
-- `ftxui.dom`
-    - `ftxui.dom.Canvas`
-    - `ftxui.dom.Deprecated`
-    - `ftxui.dom.Direction`
-    - `ftxui.dom.Elements`
-    - `ftxui.dom.FlexboxConfig`
-    - `ftxui.dom.LinearGradient`
-    - `ftxui.dom.Node`
-    - `ftxui.dom.Requirement`
-    - `ftxui.dom.Selection`
-    - `ftxui.dom.Table`
-- `ftxui.screen`
-    - `ftxui.screen.Box`
-    - `ftxui.screen.Color`
-    - `ftxui.screen.ColorInfo`
-    - `ftxui.screen.Deprecated`
-    - `ftxui.screen.Image`
-    - `ftxui.screen.Pixel`
-    - `ftxui.screen.Screen`
-    - `ftxui.screen.String`
-    - `ftxui.screen.Terminal`
-- `ftxui.util`
-    - `ftxui.util.AutoReset`
-    - `ftxui.util.Ref`
+    - `ftxui.dom`
+    - `ftxui.screen`
+    - `ftxui.util`