[Bug] Module build fails if custom SWC uses multiple entries

[kmonahan]

Describe the bug

If a custom SWC config file contains multiple entries, the custom and default SWC config are not merged together correctly. Instead of an array containing multiple SWC configs, the config becomes an object with keys '0' and '1', resulting in the error:

Module build failed (from ./node_modules/swc-loader/src/index.js):
Error: unknown field `0` at line 1 column 713

Steps to reproduce the behavior

1. Create a .swcrc file with multiple entries, such as different configuration for .js(x) versus .ts(x)
2. Attempt to build Storybook
3. Observe that the build fails with the above error

Here is the .swcrc file I started with:

[
  {
    "test": ".*\\.jsx?$",
    "env": {
      "mode": "usage",
      "loose": true,
      "corejs": "3"
    },
    "jsc": {
      "loose": true,
      "parser": {
        "syntax": "ecmascript",
        "jsx": true
      }
    }
  },
  {
    "test": ".*\\.tsx?$",
    "env": {
      "mode": "usage",
      "loose": true,
      "corejs": "3"
    },
    "jsc": {
      "loose": true,
      "parser": {
        "syntax": "typescript",
        "tsx": true
      }
    }
  }
]

I console.logged the config to try to see what was going on and saw

{
  '0': {
    test: '.*\\.jsx?$',
    env: { mode: 'usage', loose: true, corejs: '3' },
    jsc: { loose: true, parser: [Object] }
  },
  '1': {
    test: '.*\\.tsx?$',
    env: { mode: 'usage', loose: true, corejs: '3' },
    jsc: { loose: true, parser: [Object] }
  },
  env: { bugfixes: true },
  jsc: {
    transform: { react: [Object] },
    parser: { tsx: true, dynamicImport: true, syntax: 'typescript' }
  }
}

Expected behavior

I suspect that for multiple entries, the default config values need to be merged into each config item in the array rather than the array converted to an object?