feat(genai): Sample/textgen part1

[Guiners]

Description

Fixes #

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Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• Required CI tests pass (see CI testing)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

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[snippet-bot]

Here is the summary of changes.

You are about to add 5 region tags.

• genai/text-generation/textgen-async-with-txt.js:17, tag googlegenaisdk_textgen_async_with_txt
• genai/text-generation/textgen-chat-stream-with-txt.js:17, tag googlegenaisdk_textgen_chat_stream_with_txt
• genai/text-generation/textgen-chat-with-txt.js:17, tag googlegenaisdk_textgen_chat_with_txt
• genai/text-generation/textgen-config-with-txt.js:17, tag googlegenaisdk_textgen_config_with_txt
• genai/text-generation/textgen-with-mute-video.js:17, tag googlegenaisdk_textgen_with_mute_video

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[msampathkumar]

LGTM. Minor nits.