docs(reference/lint_plugins): Clarify "Testing plugins" example

[frou]

Hi there - I read this page while doing the Lint Rules Contest today and based on that have made a few small changes to fix it / make it clearer:

1. Used normal type annotations for the example linter's default export, rather than satisfies Deno.lint.Plugin (This is because I noticed when publishing my own plugin that JSR complained about the latter being a "slow type").
2. Fixed the example test file name to use a _ rather than -, so that deno test picks it up by default.
3. Mentioned that the filename we give to Deno.lint.runPlugin is just a dummy (we provide the source-code to lint as a string).
4. Fixed the assert for Deno.lint.Diagnostic.fix in the example test (it is never a function either way).
5. Fixed the link to the API documentation for Deno.lint.Diagnostic.

[Aire-One]

Hello, I'm also having trouble with this example and was preparing another similar PR. It's probably easier if we gather everything in the same place. Here are my additional quick notes 😄

[Aire-One]

Deleted a part of the example test which seems broken. It seems that the d.fix property is not yet ready for primetime, because it's not a function even when the Lint Rule has a fix configured, and currently seems to be an object like {range: undefined, text: undefined}.

The typing part was noticed before, and a PR is already merged! denoland/deno#28344

[frou]

@Aire-One Thanks, well spotted. I noticed a couple of other things needed fixed when updating that, so have pushed those too. See the edited OP.

[marvinhagemeister]

Sweet, thanks!