build: make glfw opt-in on all platforms, default to GTK on Linux

The GLFW build has grown farther and farther in feature parity with GTK
on Linux. And on macOS, the GLFW build has been unstable (crashes) for
months.

I still find the GLFW build useful for testing some core terminal work,
but it is increasingly user-hostile in case someone just downloads the
project and does a `zig build`.

I keep GLFW as up to date as I can, but the features that are missing
are due to fundamental limitations:

  - GLFW has no tabs, splits, etc. because it has no UI elements.
    I am not interested in painting UI widgets from scratch.

  - GLFW cannot support keyboard layouts robustly because it has no
    hooks to detect keyboard layouts or functions to get keymaps.

  - GLFW cannot support the Kitty keyboard protocol because the
    key/char API is too high level and it provides no low-level
    alternatives.

  - GLFW crashes on macOS under certain scenarios (this is my problem).
    I'm not interested in fixing it because the AppKit-based build
    is highly recommended.

To build or run the GLFW build you must now explicitly pass in
`-Dapp-runtime=glfw` to `zig build`.

.github/workflows/test.yml

@@ -38,7 +38,7 @@ jobs:
     # Cross-compile the binary. We always use static building for this
     # because its the only way to access the headers.
     - name: Test Build
-      run: nix develop -c zig build -Dstatic=true -Dtarget=${{ matrix.target }}
+      run: nix develop -c zig build -Dstatic=true -Dapp-runtime=glfw -Dtarget=${{ matrix.target }}
 
   build-macos:
     runs-on: macos-12
@@ -89,6 +89,12 @@ jobs:
     - name: test
       run: nix develop -c zig build test
 
+    - name: Test GTK Build
+      run: nix develop -c zig build -Dapp-runtime=gtk
+
+    - name: Test GLFW Build
+      run: nix develop -c zig build -Dapp-runtime=glfw
+
     - name: Test Dynamic Build
       run: nix develop -c zig build -Dstatic=false