chore: add support for HMR env flag

[trueadm]

Run this locally:

HMR=true pnpm run test

Interestingly, we have 21 failed tests when doing this.

[changeset-bot]

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Latest commit: 36e4c3e

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[Rich-Harris]

We should probably still allow config.compileOptions to override this — maybe do this instead?

const compileOptions: CompileOptions = {
	generate: 'client',
	rootDir: cwd,
+	dev: process.env.HMR ? true : undefined,
+	hmr: process.env.HMR ? true : undefined,
	...config.compileOptions,
	immutable: config.immutable,
	accessors: 'accessors' in config ? config.accessors : true,
	runes
};