Separate fsrx functionality into a lib

[TheBestJohn]

Add default feature to build bin
Update cli to use fsrx_lib

Bump dependencies

What:
Migrated the functionality of FSRX into a library that the CLI app uses to mark-up the text.

Why:
Moving fsrx functionality into a library would allow it to be used as a dependency as well to offer the flow state reading conversion of any text. I was looking to add fsrx into my own fork of the himalaya command line webmail client and, instead of just piping the output to fsrx, I wanted to implement it as an output mode.

Another application would be to quickly make a wasm based browser extension to manipulate <p> tags on a website

How:
I changed clap and atty to be feature dependent, added the cli feature and made it default.
I then added a lib that provides the FSRXStyler struct that maintains contrast, fixation, saccade mode and regex snippet states.
it provides publicly the style_line function which accepts a &str and returns a FSRX styled String

Checklist:

• Allow edits from maintainers option checked
• Branch name is prefixed with [your_username]/ (ex. thatvegandev/featureX)
• Documentation added
• Tests added
• No failing actions
• Merge ready
  

[TheBestJohn]

future plans: Adding a "rich-text" mode to the lib so I can introduce "falloff" in the browser plugin whereby characters get less and less strong providing a less jarring shift from emphasized to normal characters. Having this in a lib allows the cli application to not experience any changes.