fix: optimize reporter file output

[hi-ogawa]

Description

• Resolves Reporters data is bloated compared to v3.1.x #9216

This finally tries to optimize the persisted structure itself.

The raw data format is:

// files
[
  {
     id: "...",
     importDurations: {
        "imported-file-1": {
            seltTime: ...,
            totalTime: ...,
            external: ...,
            importer: ...,
         },
        "imported-file-2": {
            seltTime: ...,
            totalTime: ...,
            external: ...,
            importer: ...,
         },
     }
  },
  {
     id: "...",
     importDurations: {
        "imported-file-1": {
            seltTime: ...,
            totalTime: ...,
            external: ...,
            importer: ...,
         }
     }
  },
]

This PR squeezes this into something like:

// files
[
  {
     id: "...",
     ...
  },  
  {
     id: "...",
     ...
  },
  ...
]

// importDurations interned strings
["imported-file-1", "imported-file-2", ...]

// flat importDurations
{
  "id-1": [(index imported-file-1), (selfTime.toPrecision(3)), ..., ],
  "id-2": [...]
   ...
}

The original each key/value entry leads to about 150 bytes, but this squeezes into about 20 bytes. However, when considering the scale of the number of entries being (number of test files) x (number of source files), this approach cannot solve the inherent issues.

Result

See https://github.com/hi-ogawa/vitest-9216-repro/

• Without importDurations (feat: ability to turn off import durations collection #9262)

(test files x source files)	Blob Size
100 x 1000	239K
1000 x 1000	1.5M

• v4.0.15 (with importDurations)

(test files x source files)	Blob Size
100 x 1000	15 MB
1000 x 1000	???

• this PR

(test files x source files)	Blob Size
100 x 1000	2.2M
1000 x 1000	23M

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