clipboard: Always copy as plain text in Wayland #9737

`wl-copy` by default tries to determine the mime type of a copied bit of
text. From the [readme](https://github.com/bugaevc/wl-clipboard):

> wl-copy automatically infers the type of the copied content by running
> xdg-mime(1) on it.

So copying a Ruby script from Nvim may store it in the Wayland clipboard
as mime-type `application/x-ruby`.

This is a small reproduction without Nvim:

    $ cat test.rb
    #!/usr/bin/env ruby
    puts 'hello world'
    $ cat test.rb | wl-copy
    $ wl-paste --list-types
    application/x-ruby

This commit fixes that by telling wl-copy that all text copied from
Nvim has the mime type `text/plain`.

    $ cat test.rb | wl-copy --type text/plain
    $ wl-paste --list-types
    text/plain;charset=utf-8

1 files changed, 2 insertions, 2 deletions

diff --git a/runtime/autoload/provider/clipboard.vim b/runtime/autoload/provider/clipboard.vim
index 2fb9d74d8d..2b06ee8c48 100644
--- a/runtime/autoload/provider/clipboard.vim
+++ b/runtime/autoload/provider/clipboard.vim
@@ -73,9 +73,9 @@ function! provider#clipboard#Executable() abort
     let s:cache_enabled = 0
     return 'pbcopy'
   elseif exists('$WAYLAND_DISPLAY') && executable('wl-copy') && executable('wl-paste')
-    let s:copy['+'] = 'wl-copy --foreground'
+    let s:copy['+'] = 'wl-copy --foreground --type text/plain'
     let s:paste['+'] = 'wl-paste --no-newline'
-    let s:copy['*'] = 'wl-copy --foreground --primary'
+    let s:copy['*'] = 'wl-copy --foreground --primary --type text/plain'
     let s:paste['*'] = 'wl-paste --no-newline --primary'
     return 'wl-copy'
   elseif exists('$DISPLAY') && executable('xclip')