Add property Cef.IsShutdown #3782

[jozefizso]

Add property Cef.IsShutdown to allow checks for the shutdown state of Cef runtime (#3782)

… Cef runtime (#3782)

Fixes: #3782

Summary: Cef object will exponse the private _hasShutdown field as IsShutdown property.

How Has This Been Tested?
Manual test of the code.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Updated documentation

Checklist:

• Tested the code(if applicable)
• Commented my code
• Changed the documentation(if applicable)
• New files have a license disclaimer
• The formatting is consistent with the project (project supports .editorconfig)

[AppVeyorBot]

✅ Build CefSharp 92.0.250-CI4192 completed (commit aa348c6cbc by @jozefizso)