fmt: refactor, fix typos

[ttytm]

Pedantic refactor and typo fixes.

For typos:
There are some occurrences of splitted that were marked as typos in my case, but I left them out. It's less common than casted, but can be used in a similar manner discussed here: #18379 (comment).

Altogether, there are not too many changes, but they have been split where it seemed reasonable for easy per-commit tracking / cherry-picking.

🤖 Generated by Copilot at 138aec2

This pull request improves the code quality and readability of the vlib/v/fmt module by refactoring some logic, using match and if expressions, and fixing typos in comments and filenames. It affects the files ./vlib/v/fmt/fmt.v, ./vlib/v/fmt/struct.v, ./vlib/v/fmt/comments.v, and several test files in ./vlib/v/fmt/tests/.

🤖 Generated by Copilot at 138aec2

• Fix typos in comments and strings (link, link, link, link, link, link, link, link, link)
• Refactor if-else statements into match statements for readability and consistency in vlib/v/fmt/fmt.v and vlib/v/fmt/struct.v (link, link, link, link, link)
• Add or remove mut keyword from variables as required by V syntax or as a minor optimization and style improvement in vlib/v/fmt/fmt.v (link, link, link)
• Simplify logic for getting or updating variables by using if expression syntax in vlib/v/fmt/struct.v (link, link, link, link)
• Rename type_sym variable to sym_name for clarity and consistency in vlib/v/fmt/struct.v (link)
• Rename files with embedded comments tests to fix spelling error (link, link)

[medvednikov]

LGTM.

@yuyi98 @felipensp what do you think?