refactor: extract backend

[SukkaW]

The first PR toward fixing #226.

This PR moves all esbuild import to the backend folder. This allows future backends (such as swc or even something else) to be implemented.

The PR also specifies the interface for the backend:

export interface Backend<ExtendedTransformOptions> {
	evalTransformSync: (evalCode: string, inputType?: string) => string;
	replTransform: (code: string, filename: string) => Promise<string>;
	transformSync: (
		code: string,
		filePath: string,
		extendOptions?: ExtendedTransformOptions,
	) => Transformed,
	transform: (
		code: string,
		filePath: string,
		extendOptions?: ExtendedTransformOptions,
	) => Promise<Transformed>,
}

[SukkaW]

All CI passed, see SukkaW#1

cc @privatenumber