Make `CosmicTextSystem` Linux-only (#14728)

Since `WindowsDispatcher` requires a minimum Windows version of Windows
10 Fall Creators Update (10.0.16299), and the `alacritty_terminal`
dependency relies on conPTY, an API introduced in the same version,
additionally, `DirectWriteTextSystem` also relies on Windows 10 Fall
Creators Update (10.0.16299), so it seems reasonable to make
`CosmicTextSystem` Linux-only. And we can use `DirectWriteTextSystem` on
the Windows platform exclusively. I hope this approach makes sense.

Release Notes:

- N/A