Refine extends-to-implements code fix

[amcasey]

1. Retain surrounding trivia when swapping the keyword.
2. Insert commas at the full-starts, rather than starts, of existing
   keywords when merging with existing implements clauses.

Fixes #18794

[amcasey]

FYI @sandersn who periodically has to deal with trivia issues in code fixes and refactorings.

[ghost]

You could try something like this to avoid dealing directly with positions:

if (heritageClauses.length === 1) {
    changes.replaceNode(sourceFile, extendsToken, createToken(SyntaxKind.ImplementsKeyword));
}
else {
    changes.deleteNode(sourceFile, heritageClauses[0]);
    // Move types from 'extends' to 'implements' clause
    for (let i = heritageClauses[0].types.length - 1; i >= 0; i--) {
        changes.insertNodeBefore(sourceFile, first(heritageClauses[1].types), heritageClauses[0].types[i]);
    }
}

(and in textChanges.ts add || isExpressionWithTypeArguments(before) to the else if in getOptionsForInsertNodeBefore)

[ghost]

It was this way before, but shouldn't this only apply to heritageClauses[1] since the others will have commas already?

[amcasey]

I thought that implements I1, I2 counted as a single heritage clause (and that, in correct code, heritageClauses would have 0, 1, or 2 elements).

[ghost]

OK, but then we don't need the loop, just an if.

[ghost]

Might want to assert that this is SyntaxKind.ImplementsKeyword.

[amcasey]

My thinking was that that assertion would fail in invalid code (e.g. multiple extends clauses or an out-of-order extends clause). I'm not convinced it's worthwhile to harden a best-effort code fix against that sort of bad code (the user will already be getting a diagnostic and the change is undoable), but I don't feel strongly about it. However, I'd only add the assert if we were also adding actual checks.

[amcasey]

@Andy-MS I don't think I can use the replaceNode call you suggest because it will clobber the trivia of the replaced keyword. I did consider updating the list of types in the existing implements clause, but it wasn't clear where all the trivia would go in that case. @sandersn and I discussed it offline (before I implemented it) and concluded that this was the closest to what a user would do by hand.

[amcasey]

Good to go?