Use eslint

[ota-meshi]

What kind of changes does this PR include?

• Minor content fixes (broken links, typos, etc.)
• New or updated content
• Translated content
• Changes to the docs site code
• Something else!

Description

close #634

• What does this PR change? Give us a brief description.

This PR introduces ESLint to the project. And, add the npm script "lint:eslint" to run ESLint.
I also made some simple fixes to some of the warned files.

However, the eslint configuration may need to be changed. Currently, it is based on the recommended configuration of eslint and each eslint plugin.

• Did you change something visual? A before/after screenshot can be helpful.

There are no visual changes.

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[ota-meshi]

If you are using VS Code and ESLint extension, you can also check in the editor by adding the following settings to .vscode/settings.json.

{
  "eslint.validate": [
    "javascript",
    "javascriptreact",
    "astro",
    "typescript",
    "typescriptreact"
  ]
}

https://github.com/ota-meshi/eslint-plugin-astro#visual-studio-code

[delucis]

Hi @ota-meshi!

Thanks so much for taking the time to add this 🙌

I have a couple of questions about the config/plugin, but in general this looks great.