[Fix] `img-redundant-alt`: fixed multibyte character support

[makotot]

Fixes #969.

Is it possible to replace the word boundary \b with \s+? Although it has passed unit tests.

Also, it seemed more natural not to check word boundaries for multibyte characters, which may be a difficult point to handle also related to https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/issues/417.

(?!{) is removed because there is no difference in the unit test results whether it is there or not.

[codecov[bot]]

Codecov Report

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

Comparison is base (540cb7a) 99.01% compared to head (7058725) 99.02%.

:exclamation: Current head 7058725 differs from pull request most recent head 2d8d557. Consider uploading reports for the commit 2d8d557 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #970 +/- ## ======================================= Coverage 99.01% 99.02% ======================================= Files 105 105 Lines 1628 1634 +6 Branches 578 579 +1 ======================================= + Hits 1612 1618 +6 Misses 16 16 ```

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[ljharb]

@makotot hm, i rebased this and fixed one of the .includes usages, and now tests are failing. mind taking a look?

[ljharb]

nvm, i think i figured it out

[socket-security[bot]]

New dependencies detected. Learn more about Socket for GitHub ↗︎

Packages	Version	New capabilities	Transitives	Size	Publisher
string.prototype.includes	2.0.0	None	+0	13.8 kB	nicolo-ribaudo

[makotot]

Didn't reply due to sleep time. Thanks 🙌 I will wait for a new version to be released that includes this fix.