feat(single_char_filter): add char_only filter

[ksqsf]

Pull request

Issue tracker

Fixes will automatically close the related issue

Fixes #

Feature

Describe feature of pull request

Allow single_char_filter to keep only char candidates. Also allow toggling the filter with an option.

engine:
  filters:
    - single_char_filter
single_char_filter:
  type: char_only  # char_only | char_first
  option_name: single_char

Unit test

• [x] Done

Manual test

• [x] Done

Code Review

1. Unit and manual test pass
2. GitHub Action CI pass
3. At least one contributor reviews and votes
4. Can be merged clean without conflicts
5. PR will be merged by rebase upstream base

Additional Info

[lotem]

還有這好東西？

[ksqsf]

Not interested anymore