Suffix all imports with .js

[coyotte508]

So that we can run code in node after a basic npx tsc, without module compatibility problems

cc @martin-gorner for viz

Later we can investigate other steps needed to debug a local file, until we reduce it to the minimal amount

[martin-gorner]

source: Node v18+ docs: https://nodejs.org/api/esm.html#mandatory-file-extensions

"Mandatory file extensions
A file extension must be provided when using the import keyword to resolve relative or absolute specifiers. Directory indexes (e.g. './startup/index.js') must also be fully specified."

[martin-gorner]

I also noticed we use import statements without an extension throughout the codebase, in huggingface.js and internal projects as well. Maybe there is a rationale for that. If so, node can be made to work with extensionless imports using the "extensionless" package:
npm install extensionless and then node --import=extensionless/register test.js