Implement fluent interface into `TranslatorInterface`

[vjik]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	✔️
Fixed issues	#92

[codecov[bot]]

Codecov Report

Base: 95.25% // Head: 98.05% // Increases project coverage by +2.80% :tada:

Coverage data is based on head (b5c0f9e) compared to base (a9b034c). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #109 +/- ## ============================================ + Coverage 95.25% 98.05% +2.80% Complexity 110 110 ============================================ Files 15 15 Lines 316 309 -7 ============================================ + Hits 301 303 +2 + Misses 15 6 -9 ``` | [Impacted Files](https://codecov.io/gh/yiisoft/translator/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) | Coverage Δ | | |---|---|---| | [src/TranslatorInterface.php](https://codecov.io/gh/yiisoft/translator/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1RyYW5zbGF0b3JJbnRlcmZhY2UucGhw) | `0.00% <ø> (ø)` | | | [src/Translator.php](https://codecov.io/gh/yiisoft/translator/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1RyYW5zbGF0b3IucGhw) | `98.24% <100.00%> (+8.24%)` | :arrow_up: | | [src/CategorySource.php](https://codecov.io/gh/yiisoft/translator/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL0NhdGVnb3J5U291cmNlLnBocA==) | `93.75% <0.00%> (+10.41%)` | :arrow_up: | 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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[what-the-diff[bot]]

• Added fluent interface to TranslatorInterface
• Changed return type of methods addCategorySources(), setLocale() in Translator class from void to static
• Added test for fluent interface (testFluentInterface())

[darkdef]

I think - this changes not enough for new version. And this feature not needed for 99% cases