feat: Provide sourcemap for blocks in .vue file

[znck]

Fixes #194

[sliekens]

@znck can you take another look at this? I think this PR causes #238 and #248

[sliekens]

I'm no Rollup expert but if my config contains this:

    output: {
        file: 'dist/bundle.js',
        format: 'esm',
        sourcemap: false
    }

Then I wouldn't expect to find any source maps in dist/bundle.js. Yet if I do a search for "sourcesContent" then all my original .vue files are there.

(rollup-plugin-vue@4.2.0)

I think this needs a rewrite...