feat(line_break): add break_below_width option

[OddBloke]

Description

When specified, this option will change line_break's behaviour to only insert a line break when the width of the terminal is below the given value.

Motivation and Context

I value being able to see as much of my terminal history as possible, so my current starship configuration disables line_break. This is disadvantageous in narrow terminals, as commands I type are more likely to wrap and my right prompt may not be displayed at all.

Setting an appropriate break_below_width value allows users like me to increase the vertical space used by their prompt only when horizontal space is at a premium: I don't "lose" lines of history off the top of my screen except when doing so will give me more space to type and/or mean that my right prompt will be displayed.

Screenshots (if appropriate):

These screenshots are of the same prompt with only the width of the terminal changed:

How Has This Been Tested?

I have built starship locally with this change (on an Ubuntu 20.04 system) and tested it by:

• setting break_below_width and resizing my terminal across the boundary to observe its behaviour
• unsetting break_below_width and confirming that a line break is always emitted

Additionally, there are cargo test tests to ensure that the correct output is being emitted.

• I have tested using MacOS
• I have tested using Linux
• I have tested using Windows

Checklist:

• I have updated the documentation accordingly.
• I have updated the tests accordingly.

[OddBloke]

This change was inspired by my current custom zsh prompt, which will insert a newline if my prompt will leave me below a certain amount of space on-screen:

    PROMPT_PREFIX="<my actual prompt value>"
    # I want 20 characters of space available assuming 24 characters for RPROMPT
    MAX_PROMPT_LENGTH=$((COLUMNS-24-20))
    PROMPT="$(print "$PROMPT_PREFIX%$MAX_PROMPT_LENGTH(l.\n.)%# ")"

(The magic here is in zsh prompt escaping: %<number>(l.\n.)% means (approximately; this is my first time revisiting this part of my .zshrc in about a decade 😅) "if at least <number> characters have already been printed on this line, then emit \n otherwise emit nothing".)