chore: add _site-dev and _merged_assets to .eslintignore

[web-padawan]

What I did

Added _side-dev and docs/_merged_assets folders to .eslintignore to fix warnings I'm getting locally:

> lint:eslint
> eslint --ext .ts,.js,.mjs,.cjs .


/Users/serhii/cf/modernweb/_site-dev/guides/going-buildless/css/__mdjs-stories.js
  43:2  error  Unnecessary semicolon  @typescript-eslint/no-extra-semi

> lint:prettier
> node node_modules/prettier/bin-prettier.js "**/*.{ts,js,mjs,cjs,md}" --check --ignore-path .eslintignore

Checking formatting...
[warn] docs/_merged_data/site.cjs
[warn] Code style issues found in the above file. Forgot to run Prettier?

[changeset-bot]

⚠️ No Changeset found

Latest commit: 8a5a8c9

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[web-padawan]

Test failures are unrelated to this PR so I'll merge it.