fix(lockfile): ensure newline at EOF for lockfile (#1639)

## Description

The lockfile currently does not end with a newline at EOF. Text files
should [end with a newline](https://unix.stackexchange.com/a/18789).
This also lets you manually edit the lockfile in vim without 'fixeol'
creating a spurious change for the added newline.

This change however will create a change in users' lockfiles adding a
newline upon updating, but since the lockfile would be changing anyways
to update lazy.nvim itself, this is likely acceptable.

## Related Issue(s)

*none*

## Screenshots

*N/A*

lua/lazy/manage/lock.lua

@@ -42,7 +42,7 @@ function M.update()
       f:write(",\n")
     end
   end
-  f:write("\n}")
+  f:write("\n}\n")
   f:close()
 end