Improves HMR logic with proper parser

[natemoo-re]

Changes

• Resolves Style-only changes in .astro files don't use HMR consistently #9262
• This PR improves our HMR logic by refactoring our style detection to actually use the AST provided by the compiler.
• Previously, we were doing a best-effort attempt at determining if only a style had changed. This approach had some subtly inconsistent behavior due to the order in which modules were invalidated.
• Since our compiler now has a proper parse mode that provides a full AST, our HMR logic can be much more exact.

Testing

Tested manually, but will try to add an E2E test to replicate the issue reported

Docs

N/A, bug fixes

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[bluwy]

I think my main concern is with performance. Before, we're doing string manipulations from the compile result (that was also cached and used in transform() hook). With this PR, we're doing two parse() and AST serialization that aren't cached, which could be costly. Maybe it's worth checking the perf implications of this 🤔