JSX/TSX plugin on the TypeScript target

[danfma]

Priority: Future

Background: A long-running idea — let users write React / Solid / Vue components in C# and have them transpiled to `.tsx` / `.jsx` that ships through the framework's own compiler (SWC, Babel, Vite). This avoids building a framework-specific target for each JSX flavour; instead, the TypeScript target grows a plugin hook that knows how to emit JSX element syntax, and the framework's toolchain handles the rest.

Goal

Extend the TypeScript target with a JSX/TSX plugin path:

• A `[JsxComponent]` attribute marks a C# class or static method as a component
• A new `JsxComponentTransformer` handler lowers the component body into JSX AST nodes
• New `Tsx*` AST node kinds (`TsxElement`, `TsxAttribute`, `TsxFragment`) — parallel to the existing TS AST but with the JSX extension
• The Printer emits `.tsx` files instead of `.ts` when any component is present in a file
• Downstream: Vite / SWC / Babel + the framework's JSX plugin (Solid, React) picks it up and compiles it into framework-specific output

Scope

• `[JsxComponent]` attribute definition
• `JsxComponentTransformer` handler
• `TsxElement`, `TsxAttribute`, `TsxFragment`, `TsxSpreadAttribute` AST nodes
• Printer extensions for `.tsx` output
• Mapping C# method-body constructs to JSX (conditionals → ternaries, loops → `.map`, string interpolation → JSX text)
• One sample per framework (React, Solid) to validate the flow
• Framework-agnostic by design — the plugin emits generic JSX, the framework's own compiler is responsible for `createElement` / `h()` / `jsx()` lowering

Notes

The win is that Metano stays framework-agnostic: the TypeScript target emits JSX, and the consumer picks which framework to use via their Vite / SWC / Babel config. No need for per-framework Metano targets.

Part of #14.

[danfma]

Spec mapping: Spec change request — JSX/TSX is an extension to the TypeScript target. Would need new FRs for component lowering, JSX element syntax, and framework-agnostic emit.