shell: refactor output redirection, prepare to unify code with blissterm
Rework of !5994 (closed) without the changes on the MG and sessions.
- move stdout log code to BlissRepl class instead of having it in output wrapper classes
- use of app session (from ptpython) context
- replace output wrappers with proper BLISS output objects, default output is sys.stdout
- BlissVt100Output accepts output stream, constructor signature more similar to Vt100Output
- initialize_session executes in a greenlet, to ensure print redirection
- tests: reuse same bliss_repl context manager in shell and session tests, reorganize imports in conftest.py (with isort)