fix(SafariNomoduleFixPlugin): use RawSource instead of a plain object

[KaelWD]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information:
Writing plain objects to compilation.assets can cause the following error:

UnhandledPromiseRejectionWarning: TypeError: asset.map is not a function
    at getTaskForFile (/home/kael/Documents/vuetifyjs/vuetify-next/packages/docs/node_modules/webpack/lib/SourceMapDevToolPlugin.js:88:47)
    at /home/kael/Documents/vuetifyjs/vuetify-next/packages/docs/node_modules/webpack/lib/SourceMapDevToolPlugin.js:272:22
    at /home/kael/Documents/vuetifyjs/vuetify-next/packages/docs/node_modules/webpack/lib/Cache.js:93:5

[KaelWD]

This looks like a timing problem with the e2e tests: