feat(sv): add setup instructions to README.md

[sacrosanctic]

Closes #982

Description

Todo

• review md.ts
• review insertEnvMsg
• reonsider the structure of README.md

Checklist

• Update snapshots (if applicable)
• Add a changeset (if applicable)
• Allow maintainers to edit this PR
• I care about what I'm doing, no matter the tool I use (Notepad, Sublime, VSCode, AI...)

[changeset-bot]

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Latest commit: e9d439f

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-cli-1034-svelte.vercel.app/

[pkg-pr-new]

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npx https://pkg.pr.new/@sveltejs/sv-utils@1034

commit: e9d439f

[sacrosanctic]

@jycouet WIP, let me know if this is a good direction to take it before I go further.

[jycouet]

@jycouet WIP, let me know if this is a good direction to take it before I go further.

Thank you for starting this, yes I believe that it's going in the good direction.

A few random points

• you should create packages/sv-utils/src/tests/md.ts to test & probably iterate faster
• If there is no add-on, we should not have this ## Add-on Setup chapter? or have a message inside like "no addon selected" (to keep the anchor)
• if I run 2 times the addon, we should not have 2 times the entry
• The "Add-on Setup", is more "Add-on next step"?
• in existing project, if Add-on Setup is not there, we should maybe not update the readme at all?