This commit adds six new commands to interact with system clipboard:
- clipboard-yank
- clipboard-yank-join
- clipboard-paste-after
- clipboard-paste-before
- clipboard-paste-replace
- show-clipboard-provider
System clipboard provider is detected by checking a few environment
variables and executables. Currently only built-in detection is
supported.
`clipboard-yank` will only yank the "main" selection, which is currently the first
one. This will need to be revisited later.
Closes https://github.com/helix-editor/helix/issues/76
