custom contextSeparator fix

[vseva]

Why am I submitting this PR

Unused keys where words separated with underscore (for example "some_key", "some_key_just_started_likeprevious") not removed because hardcoded in helpers.js contextRegex and both "some_key" and "some_key_just_started_like_previous" treats like context forms of one key.

Does it fix an existing ticket?

No

Checklist

• [x] only relevant code is changed (make a diff before you submit the PR)
• [x] tests are included and pass: yarn test (see details here)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.40%. Comparing base (7e14d85) to head (168c8d5). Report is 25 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1008 +/- ## ========================================== + Coverage 95.16% 95.40% +0.24% ========================================== Files 11 11 Lines 1922 1937 +15 ========================================== + Hits 1829 1848 +19 + Misses 93 89 -4 ```

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[vseva]

@karellm please look at this PR

[karellm]

Thank you! It is now in 9.0.0