When opening a file with `File.open`, and then setting the encoding with
`IO#set_encoding`, it still correctly performs CRLF -> LF conversion on
Windows when reading files with a CRLF line ending in them (in text
mode).

However, the file is opened instead with either the `rb_io_fdopen` or
`rb_file_open` APIs from C, the CRLF conversion is _NOT_ set up
correctly; it works if the encoding is not specified, but if
`IO#set_encoding` is called, the conversion stops happening. This seems
to be because the encflags never get ECONV_DEFAULT_NEWLINE_DECORATOR
set in these codepaths.

Concretely, this means that the conversion doesn't happen in the
following circumstances:
  * When loading ruby files with require (that calls rb_io_fdopen)
  * When parsing ruuby files with RubyVM::AbstractSyntaxTree (that calls
    rb_file_open).
This then causes the ErrorHighlight tests to fail on windows if git has
checked them out with CRLF line endings - the error messages it's
testing wind up with literal \r\n sequences in them because the iseq
text from the parser contains un-newline-converted strings.

This commit fixes the problem by copy-pasting the relevant snippet which
sets this up in `rb_io_extract_modeenc` (for the File.open path) into
the relevant codepaths for `rb_io_fdopen` and `rb_file_open`.

[Bug #20101]