Rename --lp option to --mip and keep --lp as an alias

[devin-ai-integration]

Rename --lp option to --mip and keep --lp as an alias

toysat, toysolver, and toysolver_check commands have --lp option, which turns them into a mode for solving/processing MIP problems in LP files (*.lp) or MPS files (*.mps).

Initially, only the LP file was supported, so the option was named --lp. But since we now support MPS files as well, the name --mip is more appropriate.

This PR renames the option to --mip while keeping --lp as an alias for backward compatibility.

Link to Devin run: https://app.devin.ai/sessions/ef4c684380234ef1aef7399c9b139888
Requested by: [email protected]

[devin-ai-integration]

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[msakai]

I think it's not necessary to mention the deprecated option in README.md.

Could you remove the mention of the --lp option from README.md ?

[msakai]

LGTM!