Develop specifications for league sim UI and general UI design

[whatsacomputertho]

Overview

This issue will track the process of adding league sim UI requirements to the league sim spec. Some examples off the top of my head:

• MUST support creating a new league
• MUST support loading a league from a file
• MUST support writing a league to a file
• ... (possibly more)

Some more general requirements and standards may also be documented either alongside the league sim spec, or in a separate UI design spec:

• MUST support dark and light mode
• MUST be accessible
  • See the WCAG 2.1 standard
• MUST be responsive
  • Perceived latency / action-reaction
  • Wait time / render and load speed
  • "Jank" / smoothness and visual stability
  • Handling long operations (progress bars, loading icons, etc.)
• MUST be consistent
  • Consistent and well-defined color palette
  • Consistent typography and iconography
  • Consistent spacing (i.e. 4-point grid system)
  • Consistent error handling
• MAY follow a platform guideline
  • See the Apple HIG
  • See the Google material design guideline
• ... (possibly more)

Acceptance

• The league sim-specific requirements are added to the league sim spec
• The general UI design requirements are added to a new UI design spec

[whatsacomputertho]

This was partially delivered in this PR:

• feat(playback): replay and export games, start design spec fbsim-ui#18

But should not be considered complete at this point. Still more research to do on my end and still more detail to be added to the spec from that research.