Support setting "type" attribute as "text" for Atom feeds

[myl7]

Is your feature request related to a problem? Please describe.

Atom feeds support type="text" for titles and summaries.
But the current implementation always sets it to "html".

For the 3 supported values of "type", it is reasonable to drop "xhtml" since it allows complicated structure as values, which is not convenient for RSS.
But except for "html", "text" values can be common in these development, e.g., user-manually-generated feeds.
Support type="text" can make the feed fit the purpose more.

Describe the solution you'd like

I would like to request to support type="text" via, e.g., { type: "text", value: "..." } as an alternative input type of the fields title and summary.

Describe alternatives you've considered

• Leave it as is: OK... The feature is not an urgent requirement.
• Support it and type="xhtml" together: Since RSS and Atom use the same Feed class, it may be hard for RSS to decide what it should output.

Additional context

• Related RFC: RFC4287 section 3.1.1
• Related src: /src/atom1.ts L93,104