Add write method to category source

[xepozz]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	✔️

[codecov[bot]]

Codecov Report

Base: 99.62% // Head: 99.27% // Decreases project coverage by -0.35% :warning:

Coverage data is based on head (0ad55f0) compared to base (46dbefc). Patch coverage: 85.71% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #95 +/- ## ============================================ - Coverage 99.62% 99.27% -0.36% - Complexity 107 110 +3 ============================================ Files 12 13 +1 Lines 269 276 +7 ============================================ + Hits 268 274 +6 - Misses 1 2 +1 ``` | [Impacted Files](https://codecov.io/gh/yiisoft/translator/pull/95?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) | Coverage Δ | | |---|---|---| | [config/common.php](https://codecov.io/gh/yiisoft/translator/pull/95/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-Y29uZmlnL2NvbW1vbi5waHA=) | `100.00% <ø> (ø)` | | | [src/CategorySource.php](https://codecov.io/gh/yiisoft/translator/pull/95/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL0NhdGVnb3J5U291cmNlLnBocA==) | `93.75% <75.00%> (-6.25%)` | :arrow_down: | | [src/UnwritableCategorySourceException.php](https://codecov.io/gh/yiisoft/translator/pull/95/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1Vud3JpdGFibGVDYXRlZ29yeVNvdXJjZUV4Y2VwdGlvbi5waHA=) | `100.00% <100.00%> (ø)` | | 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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