[doc] Update README and user guide to use eslint flat format for TS configuration

[xiaohk]

This PR updates the TS sections in README and user-guide.md by replacing the legacy eslintrc.* configuration with the eslint flat config eslint.config.js.

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[ota-meshi]

Thank you for this PR!

I don't think we need to use tseslint.parser for *.js and *.svelte, I think it makes sense to specify only parserOptions here.

Suggested change

parser: tseslint.parser,

Maybe we need another section that only specifies parser: tseslint.parser. What do you think?