feat: Add optional linting rule to check function args and local variables have _ prefix

[alexkeating]

Description

At Scopelift we have used the convention of adding an underscore prefix for method params and local vars.

A user should be able to turn on this rule with a cli flag. It should look something like --func-params-and-local-variable-leading-underscore. If possible it would also be good to add the ability to set this rule in a custom scopelint section in the foundry.toml.