fix(msg): use python-oxmsg for MSG email parsing (#3142)
**Summary**
`partition_msg()` previously used the `msg_parser` library for parsing
Outlook MSG email files (.msg files). The `msg_parser` library is
unmaintained and has several major shortcomings such as not being able
to parse MSG files with 8-bit encoded strings and not reliably
extracting attachments.
Use the new and permissively licenced `python-oxmsg` library instead.
**Additional Context**
For reviewability purposes, this PR temporarily places the new
`partition_msg()` implementation in `new_msg.py` and references that
implementation from `msg.py`. `new_msg.py` will be renamed to `msg.py`
in a closely following PR. This avoids a very messy interleaving of
hunks in a diff between the old and re-written `partition_msg()`
implementation.
Fixes #2481
Fixes #3006
