Bug(typescript-estree): always parse mts/mjs as ESM for non-type-aware parsing about typescript-eslint

[goldentrash]

I tried to fix this issue! See #9121.
Below is a screenshot of the result.

• If the file is ESM
  

• If the file is not ESM
  

[JoshuaKGoldberg]

Editing the title because it took me a moment to figure this out. 😄

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[goldentrash]

In /tyscript-estree/src/convert.ts/Converter/convertNode, modifying sourceType when it is case SyntaxKind.SourceFile does not make sense, below is the detail.

I tried modifying the sourceFile in convertNode with extension === ts.Extension.Mjs || extension === ts.Extension.Mts, but only the AST root (SourceFile) is changed, not the multiple child Nodes.

It appears that before convertNode is called, the ts AST is fully constructed via ts.createSourceFile and then changed to the estree AST via convertNode.

[goldentrash]

In /tyscript-estree/src/create-program/createSourceFile.ts/createSourceFile, adjusting setExternalModuleIndicator, parameter of the ts.createSourceFile, doesn't make sense, below is the detail.

First, I checked how typescript's default setExternalModuleIndicator works.

1. Traverses all expressions in the source code.
2. Find ESM expressions like export.
3. Returns that Node.
4. If no nodes were found, it returns the metadata node of the SourceFile.

The default setExternalModuleIndicator is inappropriate because the intent is to parse ESM files as ESM, even if they don't have an ESM expression like export.

However, I think it would be difficult to create a proper metadata node and insert it into the SourceFile, and it would rely too strongly on an external module (typescript).

[goldentrash]

In /tyscript-estree/src/create-program/createSourceFile.ts/createSourceFile, adjusting impliedNodeFormat, parameter of the ts.createSourceFile, doesn't work, below is the detail.

The Interface annotation says "Controls the format the file is detected as", so I was expecting that, but modifying the impliedNodeFormat didn't achieve the desired result. It seems to be a parameter related to js Emit, not AST construction.