## Why?
The explanation of x and y is reversed.

https://github.com/ruby/ruby/blob/ddbd64400199fd408d23c85f9fb0d7f742ecf9e1/re.c#L251-L256
```
long
rb_memsearch(const void *x0, long m, const void *y0, long n, rb_encoding *enc)
{
    const unsigned char *x = x0, *y = y0;

    if (m > n) return -1;
```

C extension maintainers can now compile with this warning option and
the Ruby header files will generate no warnings.

[Feature #20507]

ruby_coderange_type is defined in ruby/internal/encoding/coderange.h so
we need to include it.

Previously, newline: :lf was accepted but ignored.  Where it
should have been used was commented out code that didn't work,
but unlike all other invalid values, using newline: :lf did
not raise an error.

This adds support for newline: :lf and :lf_newline, for consistency
with newline: :cr and :cr_newline.  This is basically the same as
universal_newline, except that it only affects writing and not
reading due to RUBY_ECONV_NEWLINE_DECORATOR_WRITE_MASK.

Add tests for the File.open :newline option while here.

Fixes [Bug #12436]