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

• Done

Manual test

• 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]

The library expects a Translation to be ready to query with Peek() or exhausted() in its initial state.
Not only in Next(), but also in the constructor, we need to locate the next available single-char candidate and set exhausted_ depending on whether there is any single-char candidate available. Usually I extract a function from Next and call it in both places.

The interface will be improved in r2ime. Migrating the class to C++ iterator seems to be a large change.

[ksqsf]

should be fixed. PTAL