fix(previewer): improve file_maker line splitting and timeouts

[jamestrew]

Previous method of splitting files into lines and incorporating a timer is sub-optimal in two ways

• not that fast
• since it's only checking for timeouts between each line split, huge lines can and does lead to exceeding the timeout potentially significantly (Freezes on single line files, ignoring preview timeout #3032)

By avoiding using vim.gsplit, we can avoid some overheads it needs to account for patterns and options, and we can check for timeouts more incrementally (every 200 bytes) to avoid getting caught up in huge singular lines.

Based on quick and dirty benchmarks against large-ish files in the issue above, I'm seeing about a 4-5x speed increase.