feat: add Fresh lint plugin

[csvn]

Adapts the two existing Fresh lint rules in Deno into it's own Lint plugin.

• https://docs.deno.com/lint/rules/fresh-handler-export
• https://docs.deno.com/lint/rules/fresh-server-event-handlers

Used as an example for #3174 for possible creating new Fresh lint rules too.

Note

Requires the AST regex selector fix that is included in Deno v2.4.5

TODO's

• Avoid duplicate rules when using fresh tag in Deno
• Add a "fresh/island-imports" rule to disallow importing fresh from Islands
  • We'll need to just check file path for **/islands/** or something, since it's not possible yet to configure the Island folder. I made Allow passing options to lint rules in deno.json deno#30295 to make this possible perhaps in the future

Potential future rules

• fresh/route-exports: Lint module exports of routes/*
  • Must have at least one export (not just config for RouteConfig)
  • Lint that the export is a valid one, e.g. handlers, default export, config, etc
  • Disallow exports that are "unknown", probably better to move to a non-route if it's shared
• fresh/island-props: Lint values that can be serialized correctly (may not work without type info)

Closes #2950

[csvn]

Perhaps a new package for linting is overkill, or could perhaps allow more drift (version mismatch between @fresh/lint and @fresh/core). The lint plugin could instead be a export on @fresh/core, e.g. @fresh/core/lint.

Given that it's unlikely for lint rules to require extra dependencies, it might be better to have the lint plugin just as a new export on @fresh/core. I feel like most projects will want the lint rules anyway, so maybe it's better to skip another dependency.

   {
     "lint": {
       "plugins": ["jsr:@fresh/core/lint"]
     }
   }

Note

It seems that it's not possible to use import maps with "lint.plugins" at the moment, so requires the jsr: prefix currently. I created denoland/deno#30297 to highlight this.