Add canonical samples for chat, caching, embeds, and code execution

[hsubox76]

Finish required samples (internal link: https://docs.google.com/spreadsheets/d/10X--DI0YPL3QfEMObe387rYg0t4-kuv33yKn2TIal7E/edit?resourcekey=0-kfy1Ry4YAZh6O4DltLjV9w&gid=0#gid=0)

file	region tag
chat	chat
chat	chat_streaming
chat	chat_streaming_with_images
cache	cache_create
cache	cache_create_from_name
cache	cache_create_from_chat
cache	cache_delete
cache	cache_get
cache	cache_list
cache	cache_update
embed	embed_content
embed	batch_embed_contents
code_execution	code_execution_basic
code_execution	code_execution_request_override
code_execution	code_execution_chat

Mostly trying to conform to Python: https://github.com/google-gemini/generative-ai-python/tree/main/samples

Also updated text_generation.js to use more convenient ways to display streaming and non streaming responses (reduce lines, write streaming to process.stdout to prevent line breaks).

Next PR is to add a script to add comments to each sample explaining what import lines (e.g., import { GoogleGenerativeAI } from '@google/generative-ai') will need to be added, since we cannot otherwise include import lines in the samples.

[dlarocque]

The other samples in this file END the sample before deleting the cached content. Do we want to do this here, too?

[hsubox76]

No, because this is the delete sample, so we want to show it in the sample.