Refactor dev CLI flow

[bluwy]

Changes

Note: this merges into the js-api branch

Refactor the dev command CLI flow

1. Remove openConfig usage, switch to resolveConfig instead
2. Make a somewhat cleaner dev() JS API (now accepts AstroInlineConfig)
3. Update tests with new signature.

Next will be making the same changes as no2 for all other commands, which should be easier than dev.

Testing

Existing tests should pass. I also tested the dev command locally, especially restarts when editing astro.config.mjs.

Docs

n/a. internal change for now. will document more when we expose a JS API.

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[ematipico]

Way better now!