Add type hints to all defs

[TaiSakuma]

The option disallow_untyped_defs on nextline/ shows no error.
The folder tests/ still contain untyped defs.

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (a8e16a3) 86.72% compared to head (57f654a) 86.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
- Coverage   86.72%   86.70%   -0.03%     
==========================================
  Files         112      112              
  Lines        4272     4280       +8     
  Branches      703      704       +1     
==========================================
+ Hits         3705     3711       +6     
- Misses        515      517       +2     
  Partials       52       52              

Files	Coverage Δ
nextline/continuous.py	48.07% <100.00%> (ø)
nextline/count.py	100.00% <100.00%> (ø)
nextline/disable.py	70.00% <100.00%> (+3.33%)	⬆️
nextline/fsm/factory.py	100.00% <100.00%> (ø)
nextline/main.py	79.71% <100.00%> (ø)
nextline/plugin/plugins/session/monitor.py	95.91% <100.00%> (ø)
nextline/plugin/plugins/session/spawn.py	100.00% <100.00%> (ø)
nextline/plugin/spec.py	70.11% <100.00%> (ø)
nextline/spawned/call.py	82.35% <100.00%> (+1.10%)	⬆️
nextline/spawned/plugin/plugins/filter.py	62.82% <100.00%> (ø)
... and 22 more

... and 1 file with indirect coverage changes

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