Fix flatpak packaging to a working state

This should make testing Flatpak builds a lot easier.

To build, enter `flatpak/` directory and run:

    flatpak-builder --repo=repo builddir com.mitchellh.ghostty.yml

alternatively, using org.flatpak.Builder flatpak:

    flatpak run -p org.flatpak.Builder \
      --repo=repo \
      builddir \
      com.mitchellh.ghostty.yml

The resulting flatpak can be installed using

    flatpak install ./repo com.mitchellh.ghostty

Credit of AppStream metadata goes to @yorickpeterse.

src/build/GhosttyResources.zig

@@ -202,6 +202,12 @@ pub fn init(b: *std.Build, cfg: *const Config) !GhosttyResources {
             "share/applications/com.mitchellh.ghostty.desktop",
         ).step);
 
+        // AppStream metainfo so that application has rich metadata within app stores
+        try steps.append(&b.addInstallFile(
+            b.path("dist/linux/com.mitchellh.ghostty.metainfo.xml"),
+            "share/metainfo/com.mitchellh.ghostty.metainfo.xml",
+        ).step);
+
         // Right click menu action for Plasma desktop
         try steps.append(&b.addInstallFile(
             b.path("dist/linux/ghostty_dolphin.desktop"),