fix: ignore escaped delimiters

[mdonnalley]

Ignore escaped delimiters when splitting input string for flags with delimiter and multiple

Before

my-cli --flag one,two,three` => {flag: ['one', 'two', 'three']}

After

my-cli --flag one\,two,three` => {flag: ['one,two', 'three']}

This is useful when you have flag values that contain the delimiter but shouldn't be split.

Depending on you shell, you might need to double escape to get the desired effect.

@W-16343745@
forcedotcom/cli#2928

[mshanemc]

QA: threw extra UT at it, all ✅

the breaking scenario would be "I'm already using multiple and delimiter: ',' AND I have the escape character like \, or \\, being used by my command and that's currently being interpreted as a delimiter and now it's about to not be.

seems pretty low-probability.