Update regexp for `messageCategory` validation

[rob006]

Appendix for https://github.com/yiisoft/yii2-gii/pull/511. While it fixed original intentions from https://github.com/yiisoft/yii2-gii/pull/456, it seem to be too restrictive. Even documentation proposes categories like app/error, so it does not make sense to disallow / character. In addition to that . and - could be also useful for message categories.

[what-the-diff[bot]]

• The error message for the validation of a blank message category was changed from "Message Category cannot be blank." to 'Message Category cannot be blank.'.
• The regular expression used in validating the format of a non-blank message category was updated to allow dots, dashes and slashes as well as alphanumeric characters and underscores (the original regex only allowed alphanumeric characters and underscores).
• A new error message is displayed when an invalid character is found in the inputted value: "Message Category is not valid..." instead of "...is not valid". This change also includes more information about what constitutes a correct value by listing all acceptable values rather than just stating that it should contain only certain types of characters without specifying which ones are accepted or rejected.

[bizley]

👍🏻