Add a setting to control whether to enable async jdwp

[testforstephen]

This PR relates to microsoft/java-debug#436

java.debug.settings.jdwp.async: Experimental: Controls whether the debugger is allowed to send JDWP commands asynchronously. Async mode can improve remote debugging response speed on high-latency networks. Defaults to auto, and automatically enable async mode in VS Code - Insiders.

• auto (Default)
• on
• off

[Eskibear]

Will this setting appear on settings page for vscode stable editions? If so, shall we explicitly note "for Insiders only" also in this setting's description instead of "experimental" to avoid confusion? Because for stable vscode, I also see some native "experimental" settings that works.

[testforstephen]

Will this setting appear on settings page for vscode stable editions? If so, shall we explicitly note "for Insiders only" also in this setting's description instead of "experimental" to avoid confusion? Because for stable vscode, I also see some native "experimental" settings that works.

This is not for Insiders only, just enable it on Insiders by default. For stable, you still can manually opt in.

[Eskibear]

This is not for Insiders only, just enable it on Insiders by default. For stable, you still can manually opt in.

Sorry for misunderstanding. Then it looks good to me.