text: Title: Refactoring: jobs.py
  Body: This is metaissue for list of tasks on refactoring.
  
  After #5361 I see we need to refactor jobs.py code:
  * Make singleton class like `xonsh.built_ins.XSH`
  * Maybe: add jobs as `XSH.jobs` to work with this in tests more elegant
  * Optimize methods to operate with jobs list: add function to get job by pid, to update job state or attributes.
  * Add a few tests.
  * Rename "obj" to "proc" with backwards compatibility (https://github.com/xonsh/xonsh/pull/5442)
  
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