fix: add .js extensions for NodeNext module resolution

[schickling]

Summary

This PR fixes TypeScript module resolution for consumers using moduleResolution: "NodeNext".

• Add .js extensions to all relative imports/exports in TypeScript source files
• Convert directory imports like ./lib to ./lib/index.js
• Convert from "." imports to from "./index.js"

Problem

When TypeScript processes the package with moduleResolution: "NodeNext", it cannot resolve re-exports like export * from "./renderer" because NodeNext requires explicit file extensions. This causes TypeScript to report errors like:

Module '"@opentui/core"' has no exported member 'BoxRenderable'.
Module '"@opentui/react"' has no exported member 'createRoot'.

Solution

Add explicit .js extensions to all relative imports/exports. TypeScript understands that ./renderer.js refers to ./renderer.ts during compilation, and the generated .d.ts files will have the correct extensions for NodeNext resolution.

Test plan

• Verified the fix with the reproduction at https://github.com/schickling-repros/opentui-nodenext-issue
• TypeScript trace resolution shows all relative imports are successfully resolved
• No new TypeScript errors introduced (existing errors in examples/tests remain)

Fixes #504

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[pkg-pr-new]

@opentui/core

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@opentui/react

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@opentui/solid

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@opentui/core-darwin-arm64

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@opentui/core-darwin-x64

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@opentui/core-linux-arm64

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@opentui/core-linux-x64

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@opentui/core-win32-arm64

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@opentui/core-win32-x64

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commit: e5bb983

[msmps]

@schickling mind running a quick prettier:write from the root?

[schickling]

@msmps Thanks for the review! I've resolved the merge conflict and run prettier:write as requested.

Resolved:

• Merged latest main branch changes
• Fixed conflict in Markdown.test.ts by combining both the .js extensions (for NodeNext compatibility) and the new TextRenderable import from upstream
• Applied prettier formatting

Ready for another look!