Add `onTerminalShellIntegration` activation event

[meganrogge]

This will greatly reduce latency of our terminal completions for the first request. #252329

It will also be useful for other extensions that depend on shell integration being ready - for access to executeCommand and more.

cc @Tyriar

[meganrogge]

API sync feedback:

• use :shellType from TerminalState.shell similar to "onFileSystem:pr",

/**
   * The detected shell type of the {@link Terminal}. This will be `undefined` when there is
   * not a clear signal as to what the shell is, or the shell is not supported yet. This
   * value should change to the shell type of a sub-shell when launched (for example, running
   * `bash` inside `zsh`).
   *
   * Note that the possible values are currently defined as any of the following:
   * 'bash', 'cmd', 'csh', 'fish', 'gitbash', 'julia', 'ksh', 'node', 'nu', 'pwsh', 'python',
   * 'sh', 'wsl', 'zsh'.
   */
  readonly shell: string | undefined;