Add .js extensions to echarts/zrender deep imports (ESM / echarts 6 compat)

[alidor4702]

Appends .js to the extensionless echarts/lib/... and zrender/lib/... deep imports in src/.

echarts 6 and zrender 6 added strict package exports maps ending in a "./*": "./*" catch-all, which resolve specifiers literally and no longer auto-append .js. As a result, strict-ESM bundlers (esbuild, Vite, Angular's esbuild builder) can't resolve echarts-gl's deep imports when it is consumed from source (echarts-gl/charts, echarts-gl/components, import 'echarts-gl'). Full details and a minimal repro are in #574.

The target files and their named exports are unchanged in echarts 6 / zrender 6 — only the .js extension was missing from the import specifiers. So this is a purely mechanical change:

• No runtime behavior change.
• webpack users are unaffected (webpack resolves both the extensionless and the .js form).
• Unblocks strict-ESM bundlers and echarts 6 consumers.

Verification

• An esbuild bundle of echarts-gl/charts against echarts 6.1.0 + zrender 6.1.0 resolves cleanly and registers scatter3D/grid3D (the same bundle fails with several "Could not resolve" errors without this change).
• The 3D charts render correctly in a real Angular 18 application using the esbuild builder.

Fixes #574