Implement terminal flow control

[Tyriar]

Part of #113827
Part of #74620

Remaining work:

• Create a setting disabled by default so we can merge this into master @Tyriar
• Get flow control working for serverSpawn false @Tyriar
• Get flow control working for serverSpawn true @meganrogge
• Review how flow control interacts with local processes hosted in the renderer @Tyriar
• Batch events (both from and to the ext host) @Tyriar
• Implement low and high watermark @Tyriar
• Fix tests @Tyriar
• Expose resume and pause as APIs Expose resume and pause as APIs node-pty#451 @meganrogge
• Update node-pty in vscode and remove any case on ptyProcess Update node-pty in vscode and remove any case on ptyProcess #114215 @meganrogge
• Make sure hardcoded latency is removed before merge @Tyriar