Core: add sniffs to check formatting for type declarations

[jrfnl]

1. Type declarations must have exactly one space before and after the type.
   In case of a multi-line function declaration, a new line + appropriate indentation should be used before the type declaration.
2. The nullability operator is regarded as part of the type declaration and there should be no space between this operator and the actual type.
3. Keyword-based type declarations should use lowercase.
4. Class/interface name based type declarations should use the case of the class/interface name as declared.

And specifically for return type declarations:
5. There should be no space between the closing parenthesis of the function declaration and the colon starting a return type.

These rules apply to all structures allowing for type declarations: functions, closures, catch conditions as well as the PHP 7.4 arrow functions and typed properties.

Refs:

• https://make.wordpress.org/core/2020/03/20/updating-the-coding-standards-for-modern-php/ - Type declarations
• https://developer.wordpress.org/coding-standards/wordpress-coding-standards/php/#type-declarations
• Type declarations wpcs-docs#102
• PHP docs: remove a duplication wpcs-docs#119

Refs regarding return type spacing:

• Spacing for Return Types #547
• Add ReturnType Sniff #1084
• Spacing of : for return declaration when including an argument #1420
• Fix false positive violation for return type colon #1421

Closes #547

[GaryJones]

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