Add StringValue rule

[arogachev]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	❌
Fixed issues	#594

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.10 :tada:

Comparison is base (0ee5e82) 94.69% compared to head (899c98e) 94.79%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #595 +/- ## ============================================ + Coverage 94.69% 94.79% +0.10% - Complexity 727 735 +8 ============================================ Files 85 86 +1 Lines 2204 2229 +25 ============================================ + Hits 2087 2113 +26 + Misses 117 116 -1 ``` | [Impacted Files](https://codecov.io/gh/yiisoft/validator/pull/595?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) | Coverage Δ | | |---|---|---| | [src/Rule/Length.php](https://codecov.io/gh/yiisoft/validator/pull/595?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1J1bGUvTGVuZ3RoLnBocA==) | `81.81% <ø> (ø)` | | | [src/Rule/StringValue.php](https://codecov.io/gh/yiisoft/validator/pull/595?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1J1bGUvU3RyaW5nVmFsdWUucGhw) | `100.00% <100.00%> (ø)` | | | [src/Rule/StringValueHandler.php](https://codecov.io/gh/yiisoft/validator/pull/595?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1J1bGUvU3RyaW5nVmFsdWVIYW5kbGVyLnBocA==) | `100.00% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://codecov.io/gh/yiisoft/validator/pull/595/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft)

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