Add String#each_line parameter remove_empty

[andrykonchin]

Changes:

• added remove_empty parameter for method def each_line(chomp : Bool = true, & : String ->) : Nil
• added remove_empty parameter for method def each_line(chomp = true)

Closes #15149

[andrykonchin]

I am concerned about handling "\n" lines when given chomp: false, so a line is logically "empty" but a String object returned to a user is not.

[ysbaddaden]

The line itself is still empty?

[straight-shoota]

remove_empty applies a filter, chomp defines how the items are presented.
The empty line filter should not be affected by whether non-empty lines end with a NL character or not.

[andrykonchin]

The line itself is still empty?

Confusion that may happen is what is filtered with the remove_empty filter - lines or strings as far as "\n" is a non-empty string but an empty line.

It seems logical to apply the filter to lines, that's a "\n" line will be filtered out regardless of the chomp option value. Does is make sense?