vlib, tools: make minor improvements

[ttytm]

This PR contains some accumulated changes. Cleaning stashes, I thought they can be grouped in a PR with minor improvements.
E.g.,

• preferring to do less expensive checks in if conditions first
• using a const instead of re-assigning arrays with static data in often called fns
• preferring a match statement where it can promote readability
• using loops with a predefined end if the loop logic allows for it

🤖[deprecated] Generated by Copilot at aa560c2

This pull request improves the code quality, performance, and readability of various modules in the v compiler and tools, such as vdoc, vvet, checker, parser, and comptime. It mainly involves reordering, simplifying, or removing some if statements, using match statements, and fixing typos and formatting issues. It also affects the html generation of vdoc by escaping script tags in markdown comments.

🤖[deprecated] Generated by Copilot at aa560c2

• Simplify and refactor the logic of escaping html tags in comments and collecting submodules prefixes for the navigation bar in cmd/tools/vdoc/html.v (link, link, link)
• Remove the unused markdown_escape_script_tags function from cmd/tools/vdoc/markdown.v (link)
• Move the term_colors constant from cmd/tools/vvet/vvet.v to cmd/tools/vdoc/html.v and add a new constant clean_seq to clean the tags from the comments (link, link)
• Simplify the retry logic for the ssl_do, http_do, and proxy.http_do methods in vlib/net/http/request.v by using range-based for loops instead of manual counters and while loops (link, link, link)
• Swap the order of the conditions in several if statements in vlib/v/checker/checker.v, vlib/v/checker/errors.v, vlib/v/checker/fn.v, vlib/v/checker/struct.v, vlib/v/parser/comptime.v, and vlib/v/parser/expr.v to make them more consistent and avoid unnecessary checks (link, link, link, link, link, link, link, link, link, link, link, link, link, link, link, link, link, link, link)
• Refactor the logic of registering the variables for the comptime for loop in vlib/v/parser/comptime.v by using a match statement instead of multiple if statements (link)
• Refactor the logic of setting the module name based on the language in vlib/v/parser/parser.v by using a match statement instead of multiple if statements (link)

[spytheman]

Good work.