[gen_l10n] Optionally generate list of inputs/outputs

[shihaohong]

Description

This PR creates a new option to output a list of inputs and outputs for the gen_l10n tool. It takes in the path (either relative or absolute) and creates an gen_l10n_inputs_and_outputs.json file. Using this option for the stocks app example, here is a sample output:

{
  "inputs": [
    "absolute/path/to/app/stocks/lib/i18n/stocks_en.arb",
    "absolute/path/to/app/stocks/lib/i18n/stocks_en_US.arb",
    "absolute/path/to/app/stocks/lib/i18n/stocks_es.arb"
  ],
  "outputs": [
    "absolute/path/to/app/stocks/lib/i18n/stock_strings_en.dart",
    "absolute/path/to/app/stocks/lib/i18n/stock_strings_es.dart",
    "absolute/path/to/app/stocks/lib/i18n/stock_strings.dart"
  ]
}

The PR also gratuitously fixes up terminology and tests that could be improved surrounding writeOutputFile and generateCode. These updates should not be breaking, since the developer API has not been modified.

Related Issues

Fixes #55503

Tests

I added the following tests:

• Adds a test to ensure that the json file is produced at the path specified
• Modifies existing tests to call writeOutputFile() instead of generateCode(), since generateCode has become an implementation of writeOutputFile
• Modified writeOutputFile to plural writeOutputFiles since the tool now generates more than one localization file (it has been generating more than one file prior to this PR, but the code hasn't been updated so it was confusing).

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Breaking Change

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[jonahwilliams]

Instead of using string replace, create a map and use json.encode:

_inputsAndOutputsListFile.writeAsStringSync(
  json.encode(<String, Object>{
    'inputs': _inputFileList,
    'outputs': _outputFileList,
  }),
)

[jonahwilliams]

LGTM

[HansMuller]

LGTM too, modulo a few trivial wording issues

[HansMuller]

titled => named

[HansMuller]

The value of this option is the directory...