docgen: add `additional_option` token; fix wasm-freestanding example

[jagt]

• Fix usage string --skip-code-test to --skip-code-tests.
• Added a token {#additonal_option|-rdynamic#} which introduce arbitrary flag to build-exe/obj/lib example.
• Fix wasm freestanding example, it now needs explicit export symbols to work.

This fixes #14818. The wasm freestanding shell now looks like this:

 zig build-lib math.zig -target wasm32-freestanding -dynamic -rdynamic

Cheers!