Support price directives for import

[ppetru]

When importing with an importer that generates price directives, frontent API data validation fails:

fava/frontend/src/entries.ts

Line 180 in 3551738

/** A Beancount entry, currently only support Balance, Note, and Transaction. */

It would be useful to allow importing explicit price directives.

[yagebu]

Sounds good, a PR implementing this would be welcome :)