feat: convert tabs to spaces with Style.TabWidth(int)

[meowgorithm]

Lip Gloss currently mis-measures a tab (i.e. a \t) at 0 cells wide when they actually render at different widths in different terminals (usually 8 cells, sometimes 4 cells). For theses reasons tabs are almost never what you want when designing layouts for TUIs.

With this revision, a tab will get converted to 4 spaces by default—so this is a behavioral change—but you can customize the behavior and disable it entirely with the new TabWidth(int) method.

s := lipgloss.NewStyle()        // 4 spaces per tab, the default
s = s.TabWidth(2)               // 2 spaces per tab
s = s.TabWidth(0)               // remove tabs
s = s.TabWidth(-1)              // don't convert tabs to spaces
s = s.TabWidth(NoTabConversion) // alias of the above

Disable the feature with Style.TabWidth(-1) feels a little kludgy but is perhaps okay with the addition of the NoTabConversion const. The alternative would be to have a separate method for disabling the behavior (Style.DisableTabConversion(true) or something) but what I don't love about that is that the two settings would then compete with each other.