[Bug] Regression parsing of string source maps

[mschimk1]

Describe the bug

A regression in v1.0.4 when loading source maps of type string in webpack5 instanbul loader

Steps to reproduce the behavior

Build storybook stories with webpack5 configuration. E.g.,

storybook build --webpack-stats-json

@storybook/cli v7.6.19

info => Cleaning outputDir: /storybook-static
info => Loading presets
info => Building manager..
info => Manager built (1.92 s)
info => Building preview..
info => Copying static files:
...
info Using SWC compiler
info Addon-docs: using MDX2
info => Using implicit CSS loaders
...
info => Using default Webpack5 setup

The build fails with errors similar to the one below:

Module build failed (from ../../node_modules/.pnpm/@storybook+addon-coverage@1.0.4/node_modules/@storybook/addon-coverage/dist/cjs/loader/webpack5-istanbul-loader.js):
Error: .inputSourceMap must be a boolean, object, or undefined
at transformSync (/node_modules/.pnpm/@babel+core@7.24.6/node_modules/@babel/core/lib/transform.js:42:76)
at Instrumenter.instrumentSync (/node_modules/.pnpm/istanbul-lib-instrument@6.0.2/node_modules/istanbul-lib-instrument/src/instrumenter.js:102:25)
at Object._default (/node_modules/.pnpm/@storybook+addon-coverage@1.0.4/node_modules/@storybook/addon-coverage/dist/cjs/loader/webpack5-istanbul-loader.js:36:29)

Expected behavior

The string type source map should be parsed into an object which is then fed into the code instrumentation.

Please view the patch I have contributed #44.

Screenshots and/or logs

Please refer to sample error above

Environment

• OS: MacOS Sonoma v14.5
• Node.js version: v20.12.2
• NPM version: pnpm 9.4.0
• Browser (if applicable): N/A
• Browser version (if applicable): N/A
• Device (if applicable): N/A

Additional context

A very similar issue was previously reported here #32, and fixed in #33.

However, it was reintroduced as part of Fix sourcemap generation for Webpack5 projects.