Add Timer Session Support with :TimerSession Command

[jmartinn]

Description

This pull request introduces a new :TimerSession <session_name> command to the plugin, allowing users to create and manage Pomodoro like sessions. Each session consists of a sequence of timers with predefined durations and names.

Changes

• Added: :TimerSession <session_name> command.
• Extended Configuration: Added support for session definitions in the configuration.
• Implemented Session Handling Logic: Managed the sequence of timers to run one after the other.
• Updated README: Included documentation for the new command and configuration options.

Related Issue

Closes #27

Comments

Even though I didn't write a specific test for this feature, I tried it myself and it works just fine.

Checklist

• Code formatted with StyLua.
• Code linted with luacheck.
• Wrote the corresponding tests.
• CHANGELOG updated.

[epwalsh]

This is great @jmartinn, thank you! I just made a little change to autocomplete the session name or default to the only session when no name is provided.