feat: Adds min-width parameter to enable toggling of side-by-side based on terminal width

[axsaucedo]

feat: Adds min-width parameter to enable toggling of side-by-side based on terminal width

This PR includes:

• Adding the side_by_side_min_width option with default=0
• Adding a few tests that validate the min_size applies accordingly (incl git config)
• Extended terminal width param to support TERMINAL_WIDTH_IN_TESTS

Usage tested in CLI:

git diff | /delta/target/debug/delta --side-by-side --side-by-side-min-width

Usage tested in git config:

[core]
    pager = /delta/target/debug/delta

[delta]
    side-by-side = true
    side-by-side-min-width = 140

As well as:

[core]
    pager = /delta/target/debug/delta --side-by-side --side-by-side-min-width

Functionality is as follows with term_width < min_width:

And with min_width < term_width:

Fixes #2083