cat: Add the -n line numbering feature

[PerrinJS]

Adds the -n line numbering feature to cat, with the same formatting as gnu's cat function.

[LucasChollet]

Welcome to the project!

It doesn't really matter, but you can put the function in an anonymous namespace above the main to avoid having a declaration.

And also, it feels like output_buffer_with_line_numbers can be written in a simpler way. Feel free to try something on that side, otherwise I will give it a shot on a future review.

[LucasChollet]

{: >6} will take care of putting the spaces, no need to add some before

-out("   {: >6}\t{:c}", line_count, curr_value);
+out("{: >6}\t{:c}", line_count, curr_value);

[LucasChollet]

same here

[LucasChollet]

buffer_itter == buffer_span.begin()
This condition is never true as you always increment buffer_itter on the previous line.

[LucasChollet]

This makes me have this wrong result:

$cat -n README.md 
# SerenityOS and Ladybird
        1	
        2	SerenityOS is a graphical Unix-like operating system for x86-64 computers.
...

[LucasChollet]

One issue that I can see here is that you're testing for the end of a relatively small buffer, not the end of the file. If the 32k buffer ends by \n, you will be missing a line count.

To fix that, I would take another approach:
Let's declare a struct that store the state, with the line count and a bool display_line_number.
The struct is passed to the function and also returned from it (as you did with line_count).
Then output_buffer_with_line_numbers can simply be:

for (auto const c : buffer) {
	if (display_line_number) {
		out("{: >6}\t", line_count);
		display_line_number = false;
	}
	if (c == '\n')
		display_line_number = true;
	out("{:c}", c);
}

[PerrinJS]

Yea, sorry about the bad code I was rushing as I haven't had much time this week to work on projects. Will have another go at it on Sunday. Thanks for the help though.