Optimizations for when large numbers of files are added

[tekacs]

Hullo! One common issue that I've run into with aider is that when many files are in context, the CLI slows to a crawl. Every time you hit enter, there's a very long delay. This is also an issue in watch mode.

This PR fixes this issue, by showing a summary view and skipping the lag of tokenization when displaying the CLI with large numbers of files in context.

Having run with this for a few days, I can attest that input latency becomes much much more pleasant with this change!

[CLAassistant]

All committers have signed the CLA.