compose: always use cr-lf line endings

.eml files are universally formatted with DOS line-endings, per RFC5322.
Make sure to replace line endings with CRLF before sending the message.
Always present CRLF files to the text editor. If the editor forces LF
line endings, convert them back to CRLF.

Text editors (vim and nvim at least) detect the .eml extension and force
dos line endings. Emacs seems not to care and will use whatever line
endings are present in the file. Kakoune seems to force unix line
endings and does not care about the contents of the file.

Notes:

* add explicit calls to Truncate() to ensure any trailing content is
  stripped from the file when overwriting with shorter content.

* NewCRLFReader cannot be reused since we need buffered and unbuffered
  versions of it (for GetBody and {Set,append}Contents, respectively).

Link: https://datatracker.ietf.org/doc/html/rfc5322
Link: https://github.com/vim/vim/blob/v8.2.3583/runtime/ftplugin/mail.vim#L29-L35
Link: https://github.com/neovim/neovim/blob/v0.6.0/runtime/ftplugin/mail.vim#L29-L35
Signed-off-by: Robin Jarry <robin@jarry.cc>
Tested-by: Bence Ferdinandy <bence@ferdinandy.com>
Tested-by: Koni Marti <koni.marti@gmail.com>

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