**Summary**
Install new `@apply_metadata()` on TXT.
**Additional Context**
- Both EML and MSG delegate to both HTML and TXT to partition the
message-body, depending on which MIME-part body payload is selected
(`text/plain` or `text/html`). This PR prepares the way to remove
decorators from EML and MSG in the next PR.
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Co-authored-by: ryannikolaidis <1208590+ryannikolaidis@users.noreply.github.com>
Co-authored-by: scanny <scanny@users.noreply.github.com>
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