checker: extend byte deprecation warnings to return types

[ttytm]

🤖 Generated by Copilot at 1cde7b0

Refactored string and json modules to use u8 instead of byte and added a deprecation warning for byte. This is part of a transition to remove the byte alias and use u8 consistently.

🤖 Generated by Copilot at 1cde7b0

• Change the return type of string.at from byte to u8 and move the unsafe block inside the return statement (link, link)
• Move decode_byte from vlib/json/json_primitives.v to vlib/builtin/string.v and use u8 instead of byte (link)
• Add a warning to the checker for using byte instead of u8 (link)

[spytheman]

Good work.