Fix IndexError in `Strings::Wrap.wrap` and correct ANSI color insertion on wrap

[onk]

Describe the change

When wrapping lines, reset the insertion index of carried-over ANSI codes to 0 so that they are always applied at the start of the new line.

This probably fixes #4 and #16.

Why are we doing this?

1. Prevent IndexError

When an ANSI code appeared at the end of a line, and the wrapped remainder of the text was shorter, the code was reinserted at an out-of-range position on the next line, causing an IndexError. Resetting the insertion position resolves this issue.

Repro:

Strings::Wrap.wrap("aaaaaaa \e[31mbb", 8)
# /Users/.../strings/wrap.rb:158:in `String#insert': index 8 out of string (IndexError)
#   from ... Strings::Wrap.wrap

2. Preserve ANSI stack when reapplying colors

insert_ansi kept using the original insert index from the previous line, so carry-over codes without resets were reinserted at the wrong position. Resetting every pending entry to [code, 0] to be applied at the line start, which prevents misplaced inserts and lets nested codes line up correctly.

Repro:

Strings::Wrap.wrap("aaaa \e[31mbbbb \e[32mcc", 5)

Expected:

aaaa 
\e[31mbbbb \e[0m
\e[31m\e[32mcc\e[0m\e[0m

Actual:

aaaa 
bbbb 
cc\e[0\e[32m\e[31mm\e[0m

Benefits

• Prevents IndexError caused by reinserting ANSI codes past string boundaries
• Ensures ANSI color state is reapplied consistently across wrapped lines

Drawbacks

Requirements

• [x] Tests written & passing locally?
• [x] Code style checked?
• [x] Rebased with master branch?
• [x] Documentation updated?
• [x] Changelog updated?