feat(bigfile): take into account lines length

[chrisands]

Added check for long lines
#372

[folke]

Sorry, but this is a big no for performance.
I'm willing to accept a PR that checks the length of the first buffer line vim.api.nvim_bug_get_lines,
but not like how you did it.

That filetype check will run very frequently, so loading files inside of it is not something I want to see here

[folke]

And please change it to line_length

[chrisands]

From issue

It may be slow to find and count the max line length, in which case we could also allow configuring the maximum number of lines to search from the top. Note that we can't always assume the first line will be long (e.g. it could be a short license comment with the long minified lines further down).

@folke is it okay to make configurable how many lines it can scan?
I would like to make minimum two lines since first line can be short license line and second be the long one.