feat(commands): add :Preview open subcommand

Problem: after closing a viewer, there was no way to re-open the last
compiled output without recompiling.

Solution: track the most recent output file per buffer in a `last_output`
table that persists after compilation finishes. Add `compiler.open()`,
`M.open()`, and wire it into the command dispatch.

spec/commands_spec.lua

@@ -35,6 +35,13 @@ describe('commands', function()
       end)
     end)
 
+    it('does not error on :Preview open', function()
+      require('preview.commands').setup()
+      assert.has_no.errors(function()
+        vim.cmd('Preview open')
+      end)
+    end)
+
     it('does not error on :Preview toggle with no provider', function()
       require('preview.commands').setup()
       assert.has_no.errors(function()