feat: stop lsp clients plugin

[josh-nz]

This PR extracts the stopping of the LSP clients from the delete_buffers plugin to a separate plugin.

It does introduce possible breaking changes. If a user has delete_buffers set to true, then the LSPs will no longer be stopped since the plugin config defaults to false (which is the same default as delete_buffers).

If we change the LSP default config to true, then it breaks behaviour for those users with delete_buffers set to false. So there is no win.

We could implement a possible config patch if this is desirable. Given the default Neovim behaviour is to do nothing with attached LSPs when you normally delete buffers, I think the default of false is sufficient, and there is no need for a config patch. But the final decision is with you on how to proceed.

Closes #66

[jedrzejboczar]

Looks good, thanks for implementing this. I think we can merge it as is, as a breaking change, there is no simple way to prevent it from changing behaviour for some users.