fix(dsv): update type of processRow option

[jamesscottbrown]

Rollup Plugin Name: dsv

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

If yes, then include "BREAKING CHANGES:" in the first commit message body, followed by a description of what is breaking.

List any relevant issue numbers: #1492

Description

This updates the type definition for the processRow option to the DVS plugin, so that:

• a function that doesn't return anything can be used
• a function that returns a row object that includes non-string values (such as number or dates) can be used

[shellscape]

Thanks for the PR. https://github.com/rollup/plugins/blob/master/packages/dsv/test/types.ts will need to be updated to consider the changes you've made here.

[shellscape]

@jamesscottbrown ping. if this reaches 60 days without a response, it'll be closed as abandoned.