@flue/vite: parse JSX markdown importers and delegate external skill registries

[jairojair]

Package

@flue/vite@2.0.3 with Vite 8.x.

Reproduction

1. Create a .tsx module containing a type-only import and JSX, and a static
   .md import:

   import type { Props } from './props';
   import instructions from './instructions.md';
   
   export const view = <div>{instructions}</div>;

2. Build with the Flue Vite plugin.

The markdown transform calls parserLangForFile('view.tsx'), but the current
implementation returns js. The parser then rejects valid TSX syntax.

A second independent reproduction occurs when another Vite plugin owns a
virtual skill registry. Given an importer ID such as
\\0agents:skills:/workspace/src/skills that reaches a resolved
/workspace/src/skills/example/SKILL.md, Flue's markdown resolver throws
Skill import ... reached resolution untransformed instead of delegating the
edge to the plugin that owns the virtual namespace.

Expected behavior

• .tsx and .jsx files are parsed with the corresponding JSX-aware parser.
• A SKILL.md import from an explicitly external virtual registry is left for
  that registry's resolver; Flue should keep rejecting genuinely untransformed
  Flue imports from ordinary JS/TS importers.

Proposed fix

The smallest fix is to map .tsx → tsx and .jsx → jsx in
parserLangForFile, and return null from the markdown resolver only when the
importer starts with \\0agents:skills:. A focused test can cover both cases
without depending on a particular application.

The current main branch (832ad2ee, published as 2.0.3) still reproduces both
failures.