Add shellIntegrationNonce support to TerminalOptions and ExtensionTerminalOptions

[Copilot]

This PR implements support for custom shell integration nonces in VS Code terminals, allowing extensions to enable shell integration features for shells that VS Code doesn't natively support.

Problem

Previously, the VSCODE_NONCE environment variable was only available for VS Code's natively supported shells (bash, zsh, pwsh, etc.). This prevented extensions like Julia from implementing shell integration features because:

1. Shell integration injection would fail for unsupported shells
2. No VSCODE_NONCE environment variable would be set
3. Extensions couldn't send trusted shell integration sequences

Solution

Added a new optional shellIntegrationNonce property to both TerminalOptions and ExtensionTerminalOptions interfaces, enabling extensions to provide their own nonce for shell integration.

API Changes

export interface TerminalOptions {
  // ... existing properties
  shellIntegrationNonce?: string;
}

export interface ExtensionTerminalOptions {
  // ... existing properties  
  shellIntegrationNonce?: string;
}

Usage Example

// Extensions can now enable shell integration for any shell:
const terminal = vscode.window.createTerminal({
  name: 'Julia REPL',
  shellPath: '/usr/bin/julia',
  shellArgs: ['-i'],
  shellIntegrationNonce: 'custom-nonce-12345'
});

// The VSCODE_NONCE environment variable will be available
// in the terminal, allowing shell integration sequences:
// print("\x1b]633;A;$(ENV["VSCODE_NONCE"])\x07")

Implementation Details

1. API Extension: Added shellIntegrationNonce to VS Code's public API
2. Flow Implementation: Nonce passes through ExtHost → MainThread → Terminal Process
3. Critical Fix: Modified terminal process to set VSCODE_NONCE environment variable even when shell integration injection fails for unsupported shells
4. Security: Maintains nonce verification model while extending to all shells

Benefits

• ✅ Extensions can implement shell integration for any shell type
• ✅ Works with both regular terminals and pseudoterminals
• ✅ Maintains security through nonce verification
• ✅ Backward compatible - existing code unchanged
• ✅ Enables rich terminal features (command detection, decorations, etc.) for custom shells

This enables extensions like Julia to provide the same shell integration capabilities that were previously only available for VS Code's natively supported shells.

Fixes #190253.

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