`text::analyze` does not report a trailing mandatory break

[dhardy]

If the input text to text::analyze terminates with 0xa (line_break=LF), this mandatory break is not reported.

This is unimportant where whitespace may be trimmed, but an issue where with text editors where a trailing new-line must have a position.

Suggested fix: add fn is_mandatory_break to LineBreak and document that users of analyze should check this for the last char. (I'm not 100% sure this works with multi-byte breaks line \r\n, but I think here \r would not be reported so it would be fine.)

[dhardy]

I believe the following is sufficient:

let hard_break = match props.line_break() {
    LB::BK | LB::CR | LB::LF | LB::NL => true,
    _ => false,
};