Support sleep_before, sleep_after

[tony]

Requires #751 is fixed

#748

Example: shell_command_before + sleep

Todo

Fix and test shell_command_before

``` Traceback (most recent call last): File "/home/t/work/python/tmuxp/.venv/bin/tmuxp", line 5, in cli.cli() File "python3.10/site-packages/click/core.py", line 1128, in __call__ return self.main(*args, **kwargs) File "python3.10/site-packages/click/core.py", line 1053, in main rv = self.invoke(ctx) File "python3.10/site-packages/click/core.py", line 1659, in invoke return _process_result(sub_ctx.command.invoke(sub_ctx)) File "python3.10/site-packages/click/core.py", line 1395, in invoke return ctx.invoke(self.callback, **ctx.params) File "python3.10/site-packages/click/core.py", line 754, in invoke return __callback(*args, **kwargs) File "python3.10/site-packages/click/decorators.py", line 26, in new_func return f(get_current_context(), *args, **kwargs) File "/home/t/work/python/tmuxp/tmuxp/cli.py", line 1132, in command_load load_workspace(config[-1], **tmux_options) File "/home/t/work/python/tmuxp/tmuxp/cli.py", line 656, in load_workspace sconfig = config.expand(sconfig, os.path.dirname(config_file)) File "/home/t/work/python/tmuxp/tmuxp/config.py", line 308, in expand session_config["shell_command_before"] = expand_cmd(shell_command_before) File "/home/t/work/python/tmuxp/tmuxp/config.py", line 200, in expand_cmd cmds[cmd_idx]["cmd"] = expandshell(cmds[cmd_idx]["cmd"]) KeyError: 'cmd' ```

Demo

session_name: 'glorp'
# Reference this issue for why we need to use `source` here:
# https://github.com/tmux-python/tmuxp/issues/412
shell_command_before:
- cmd: source $(poetry env info --path)/bin/activate
  sleep_before: 1
  sleep_after: 1
windows:
  - panes:
    - echo "hello"

You'd this this would be possible

session_name: 'glorp'
# Reference this issue for why we need to use `source` here:
# https://github.com/tmux-python/tmuxp/issues/412
windows:
  - panes:
    - shell_command:
      - cmd: source $(poetry env info --path)/bin/activate
        sleep_after: 1
      - echo "hello"

[codecov]

Codecov Report

Merging #750 (89bfa9e) into master (5bfb779) will increase coverage by 0.01%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #750      +/-   ##
==========================================
+ Coverage   76.07%   76.08%   +0.01%     
==========================================
  Files           7        7              
  Lines        1187     1196       +9     
  Branches      312      314       +2     
==========================================
+ Hits          903      910       +7     
- Misses        197      198       +1     
- Partials       87       88       +1     

Impacted Files	Coverage Δ
tmuxp/workspacebuilder.py	85.07% <77.77%> (-0.35%)	⬇️

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