Experiment with buffering the pty data to improve performance

[Tyriar]

Related: #53974

[Tyriar]

For a terminal 140x66 on my Linux box (canvas_2d disabled)

Before buffering time ls -lR . was ~22s

After buffering time ls -lR . was ~8s (before #53974 it was around 7-8s, note this is with the stable minified build)

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140x19

Before: 4s

After: 4s

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The gains appear to mostly be beneficial when rendering takes longer

[Tyriar]

On my macbook the differences seem minimal, going to hold off on this for now.

[Tyriar]

Closing, ext host buffering tracked in #48513 and this would be more relevant if the proc was out of the renderer (#74620).