Refactoring: make subproc_captured_* methods private and provide xonsh API instead

[anki-code]

I noticed that another projects (e.g. conda, xontrib-prompt-bar) are using xonsh.built_ins.subproc_captured_* methods to run commands using xonsh. But behavior of this methods is strongly related to parser (i.e. substitutions), environment (i.e. $XONSH_SUBPROC_OUTPUT_FORMAT) and CommandPipeline (i.e. a bunch of settings) and these methods were originally used in parser. If we change environment or parser we can break other projects (mostly integrations and xontribs) downstream code.

This is why we need to separate subproc_captured_* into:

• the functions that are used in between of parser and CommandPipeline.
• the functions that are atomic public xonsh API to run commands and get output

For example subproc_captured_stdout now can return str or list (#5377) because we need this behavior in parser as well as in CommandPipeline but it's uncomfortable behavior for xonsh API users who expect to have str forever.

For community

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