feat: Add support for masked password prompt

[TomPlanche]

Summary

Adds an optional mask character to the Password prompt. By default the input stays fully hidden (nothing echoed) exactly as before; when a mask is set, one mask character is echoed per typed character.

Resolves #340.

let password = Password::new()
    .with_prompt("Enter password")
    .with_mask('*') // if not set, input stays fully hidden
    .interact()?;

Changes

• New with_mask(char) builder method on Password, backed by a mask: Option<char> field (None by default).
• When a mask is set, input is read key-by-key via Term::read_key() and a mask character is echoed for each printable character. Backspace erases the last character; Enter submits. This path mirrors read_secure_line's trailing-newline behaviour so both code paths behave uniformly.
• When no mask is set, behaviour is unchanged (read_secure_line).
• Extended examples/password.rs to demonstrate the masked prompt.

Notes

• Non-breaking: purely additive. The new Option<char> field defaults to None, preserving the current fully-hidden behaviour.
• Zeroize: read_masked_line returns a String and the caller wraps it in Zeroizing in interact_on, identical to the existing read_secure_line path, so the existing memory-scrubbing guarantee is unchanged.

[djc]

Looks okay modulo one nit. Please squash your commits.

[djc]

Thanks!