use JSON.stringify in caching parse results

[Tidyzq]

In order to fix issue: vuejs/rollup-plugin-vue#239
The reason is that rollup will manipulate the styles.map.mapping object, which is in https://github.com/rollup/rollup/blob/ea82d134400ecee72e274a02b0e623959e1b36a8/src/utils/transform.ts#L30

if (originalSourcemap && typeof originalSourcemap.mappings === 'string')
	originalSourcemap.mappings = decode(originalSourcemap.mappings);

So we should return a fresh copy by JSON.stringify and JSON.parse when the cache is hit.

[znck]

The output SFC descriptor should be treated immutable. Rollup plugin should create a copy of descriptor rather than manipulating it directly.