Rewrite "Export grid data as CSV" to use DownloadHandler

[HerbertsVaadin]

Description

Replaced addon usage with native DownloadHandler usage.
Please list all relevant dependencies in this section and provide summary of the change, motivation and context.

Fixes #394

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
  • Not relevant
• New and existing tests are passing locally with my change.
  • Not relevant
• I have performed self-review and corrected misspellings.