Require confirmation before closing a pinned tab

[Eliezergh]

Description

This pull request introduces a confirmation dialog whenever a user attempts to close a pinned tab, regardless of the method used (close button, middle-click, ⌘W, or quitting the app).

The confirmation dialog clearly identifies the tab as pinned to avoid accidental closure.

Builds on ed3f5b0 ("Add pinned tabs support") — pinned tabs had no close protection, allowing them to be closed the same as any other tab.

Changes

Pinned Tab Close Confirmation:

• Added a new close reason, tabIsPinned, to iTermPromptOnCloseReason for use in close confirmation dialogs. [1] [2]
• Updated the tab close confirmation logic in PseudoTerminal.m to prepend "pinned" to the tab identifier in confirmation dialogs, making it clear when a pinned tab is being closed.
• Modified the prompt-on-close reason logic to include a reason if any open tab is pinned, ensuring the prompt is shown when quitting with pinned tabs open.
• Enforced confirmation for closing pinned tabs via the tab close button or middle-click, preventing accidental closure without explicit confirmation.
• Updated documentation to reflect that closing a pinned tab now always requires confirmation, with the dialog clearly indicating the tab is pinned.
• Added a release note to notes-3.7.txt.