Fix validation for `messageCategory`

[rob006]

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

Previous regexp matched strings like <div>.

[what-the-diff[bot]]

• The regex pattern for messageCategory validation was changed from \w+ to ^\w+$
• This change will make the generator throw an error if a user tries to use any special characters in their message category name, such as '.' or '/'

[bizley]

Thanks