lock: write lock file with final newline (#1057)

* lock: write lock file with final newline

Before this commit, `nimble lock` wrote the lock file without a final
newline:

    $ nimble lock
         Info:  Generating the lock file...
      Verifying dependencies for foo@0.1.0
      Success:  The lock file is generated.
    $ cat nimble.lock
    {
      "version": 1,
      "packages": {}
    $ wc -l nimble.lock
    3 nimble.lock

Fixes: #1056

* Update src/nimblepkg/lockfile.nim

---------

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

src/nimblepkg/lockfile.nim

@@ -36,7 +36,9 @@ proc writeLockFile*(fileName: string, packages: LockFileDeps,
       $lfjkPackages: packagesJsonNode
       }
 
-  writeFile(fileName, mainJsonNode.pretty)
+  var s = mainJsonNode.pretty
+  s.add '\n'
+  writeFile(fileName, s)
 
 proc readLockFile*(filePath: string): LockFileDeps =
   {.warning[UnsafeDefault]: off.}