feat: add user-configurable font family, font size, and initial command for xterm sessions (#13)

[Akash-nath29]

This pull request implements and documents the feature request in #13, allowing users to customize the font family, font size, and initial shell command for colab-xterm sessions.

Key changes:

• Backend (notebook magic):

  • Updated %xterm magic in notebook.py to accept fontfamily, fontsize, and command as arguments (e.g. %xterm port=8080 fontfamily="consolas" fontsize=18 command="top").
  • These arguments are parsed and passed to the frontend as query parameters.
  • The backend launches the requested initial shell command if provided.

• Frontend:

  • index.ts now reads fontfamily and fontsize from the URL query parameters and uses those settings when constructing the xterm.js terminal.
  • Defaults remain (fontfamily=monospace, fontsize=14) if parameters are not provided.

• CLI/Local Usage:

  • While CLI arguments for font are not supported, users can now manually append ?fontFamily=...&fontSize=... to the local URL to preview different terminal styles.

Usage examples:

• In a notebook cell:

  %xterm port=8080 fontfamily="courier" fontsize=16 command="htop"
  

• In a browser:

  http://localhost:8080/?fontfamily=Fira%20Mono&fontsize=16
  

Other improvements:

• Fixed subprocess launching to use the correct Python executable with sys.executable, which improves compatibility across environments.
• Updated documentation and comments for maintainability.

---

Closes: #13