Consistent, predictable json/json5 handling

[rarkins]

Describe the proposed change(s).

Files with extension .json should use JSON.parse(), files with extension .json5 or without extension such as .renovaterc should use JSON5.parse().

Where we are allowing JSON5 for .json files today, we should change so that we warn if a .json file parses with JSON5 but not with JSON.

Then after sufficient time has passed, perhaps 2 major releases, we should remove JSON5 parsing of .json files, which will result in a Config Validation error.

We should do the same with presets too.

[viceice]

sounds good to me

[rarkins]

An alternative: allow VSCode-style JSONC for .json files: https://github.com/microsoft/node-jsonc-parser

But some context: microsoft/vscode#100688

[viceice]

interesting library. maybe it allows us to migrate config files with less changes

[rarkins]

It's all quite a mess - there's also an argument for saying if you want JSONC then use .jsonc. I think these days we can support almost unlimited json/jsonc/json5 extensions for each file name anyway.

[RahulGautamSingh]

export function parseJsonWithFallback(content: string): any {
  let parsedJson: any;

  try {
    parsedJson = JSON.parse(content);
  } catch (err) {
    try {
      parsedJson = JSON5.parse(content);
      logger.warn('JSON5.parse was used to parse the JSON data. Please check your json file');
    } catch (err) {
      return null; 
    }
  }

  return parsedJson;
}

This should work, I think.
Will add this function to utils folder aand use wherever necessary(JSON5 used to parse json file).

[rarkins]

Yes, this looks good, although we maybe want to log if both parsing fails too

[RahulGautamSingh]

I think throwing an error will be better, incase the content isn't parsable as that is current behaviour wherever parsing is performed.

[jamietanna]

Sounds reasonable to require JSON in .json and require JSONC/JSON5 in a different filename, it's always nicer to have this sort of thing be extra predictable, as well as nudging editors what the correct file format is.

I assume that optimizeForDisabled and other config will be updated, if needed, to handle the new filename(s)?

[rarkins]

We should use the onboardingConfigFileName value for optimizeForDisabled. This probably should be renamed too, so that it's more generic.