use textwidth for string display truncation

[stevengj]

This uses the new textwidth-based string-truncation functions from #55351 (with some additional refactoring) for truncating strings in the 3-argument show, replacing the codepoint-based method implemented by @StefanKarpinski in #40736.

It makes a big difference when displaying strings that have width-2 or width-0 characters. For example, in the previous implementation, you got very unequal-length displays for different character widths:

julia> "x"^1000 # width-1 chars
"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx" ⋯ 459 bytes ⋯ "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"

julia> "x̂"^1000 # contains width-0 chars
"x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂" ⋯ 2190 bytes ⋯ "x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂"

julia> "🍕"^1000 # width-2 chars
"🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕" ⋯ 1840 bytes ⋯ "🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕"

Now, you get:

julia> "x"^1000 # width-1 chars
"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx" ⋯ 460 bytes ⋯ "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"

julia> "x̂"^1000 # contains width-0 chars
"x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂" ⋯ 1378 bytes ⋯ "̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂x̂"

julia> "🍕"^1000 # width-2 chars
"🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕" ⋯ 2920 bytes ⋯ "🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕🍕"

(They look more equal in my terminal. In my browser, 🍕 is slightly more than 2 columns wide 😕.)