W-19653425: Remove post-build.js from build process

[kylewalke]

What does this PR do?

• Removed scripts/post-build.js and integrated its functionality directly into tsup.config.ts
• Updated VSCode extension build pipeline to use tsup's onSuccess callback for post-build tasks

The post-build script previously handled three key tasks after bundling:

1. Copying worker files from apex-ls/dist to extension/dist
2. Copying manifest files and resources to dist directory
3. Fixing package.json paths for VSCode extension loading

These tasks are now executed via the executePostBuildTasks() function called through tsup's onSuccess hook, eliminating the external script dependency while maintaining identical functionality.

What issues does this PR fix or reference?

@W-19653425@