feat: add diff and preview show intents

[idursun]

This change adds show actions to diff and ui.preview, allowing custom actions to populate those panels with the output of commands they run themselves. It restores the old “send command output to diff” workflow.

Example:

config.action("show diff summary in diff", function()
  local output = jj("diff", "-r", context.change_id(), "--summary", "--color", "always")
  diff.show(output)
end, {
  key = "shift+x",
  scope = "revisions",
})

config.action("show diff summary in preview", function()
  local output = jj("diff", "-r", context.change_id(), "--summary", "--color", "always")
  ui.preview.show(output)
end, {
  key = "ctrl+shift+x",
  scope = "revisions",
})

Internally, built-in diff loading was also moved onto the same diff.show path, and show now opens the target panel automatically if needed.

fixes #579
fixes #589

#587

[baggiiiie]

nice!

[idursun]

had to rebase to resolve conflict