dist: use valid application id in nvim.appdata.xml #22046

According to [AppStream spec](https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-id-generic),
the <id> element should contain a reverse-DNS scheme:

    {tld}.{vendor}.{product}

Since the flathub requires that, the flatpak build replaces `<id>nvim</id>` with
`<id>io.neovim.nvim</id>`. That results in ID mismatch between flatpak version
and version from distribution's repositories. Because of that, software stores
are displaying two different neovims, instead of one neovim with options to
download it either from flatpak or from distribution's repos. We can use the
`<provides><id>nvim</id></provides>`, for everyone who was expecting the id to
be `nvim`.

runtime/nvim.appdata.xml

@@ -7,7 +7,7 @@
     https://github.com/flathub/io.neovim.nvim
 -->
 <component type="desktop-application">
-  <id>nvim</id>
+  <id>io.neovim.nvim</id>
   <translation type="gettext">nvim</translation>
   <project_license>Apache-2.0</project_license>
   <metadata_license>CC0-1.0</metadata_license>
@@ -61,5 +61,6 @@
   <url type="translate">https://github.com/neovim/neovim/tree/master/src/nvim/po</url>
   <provides>
     <binary>nvim</binary>
+    <id>nvim</id>
   </provides>
 </component>