Clarify tab handling with enum API

[ainame]

Summary

• replace the ambiguous tabWidth option with tab: DisplayWidth.Tab
• support both terminal-style tab stops and fixed-space tab counting
• add tests and diagram-style docs to clarify the behavior difference

Details

• DisplayWidth.Tab.tabStops(n) advances to the next multiple of n
• DisplayWidth.Tab.fixedSpaces(n) counts each tab as exactly n columns
• tab: nil keeps the previous default behavior where is measured as width 0
• stripsANSI and tab still affect string measurement only

Testing

• swift test