codegen: keep code coverage comments

[Boshen]

Upstream: vitest-dev/vitest#2021

There was also a request in the discord channel:

Istanbul ignores: https://github.com/istanbuljs/nyc?tab=readme-ov-file#parsing-hints-ignoring-lines
C8, V8 and node:coverage ignores: https://github.com/istanbuljs/v8-to-istanbul/blob/46b972458b40ea0d628c80dd60223cc909f81cd0/lib/source.js#L44-L81

These are poorly documented, and the ecosystem doesn't have conventions around this. Atm I'm building parser that can pick this regexp: https://github.com/AriPerkkio/ast-v8-to-istanbul/blob/f1424941cb3b1197b0a9a061b285fdc1b818e220/src/ignore-hints.ts#L13-L14 - but there's also the start, stop next 7 comments in original v8-to-istanbul.
I'd recommend to use this as reference in test cases. For example esbuild fails to pass legal comments that are between if and else nodes. It works as intended, but isn't great for testing tools. https://github.com/istanbuljs/istanbuljs/blob/main/packages/istanbul-lib-instrument/test/specs/if-hints.yaml

[AriPerkkio]

Example comments that should be preserved:

/* v8 ignore next */
/* v8 ignore next 2 */
/* v8 ignore start */
/* v8 ignore stop */
/* v8 ignore if */
/* v8 ignore else */
/* v8 ignore file */

/* c8 ignore next */
/* c8 ignore next 2 */
/* c8 ignore start */
/* c8 ignore stop */

/* node:coverage disable */
/* node:coverage enable */
/* node:coverage ignore next */
/* node:coverage ignore next 2 */

/* istanbul ignore if */
/* istanbul ignore else */
/* istanbul ignore next */
/* istanbul ignore file */

• All of these comments should support inline comment too, e.g. // v8 ignore next.
• All of these comments should support additional suffixes, e.g. /* v8 ignore next -- some explanation why this is ignored */

Some example edge cases that esbuild fails to preserve. I'm sure there are more, but here's just couple of examples:

Comment between IfStatement's consequent and alternate

• https://esbuild.egoist.dev/#W1siaW5k...

function something(arg) {
  if (arg === 1) {
    return 1;
  } /* istanbul ignore next -- @preserve */ else if (arg === 2) {
    return 2;
  } else {
    return 3;
  }
}

something(1);
something(3);

Comment between TryStatement's block and handler:

• https://esbuild.egoist.dev/#W1siaW5...

try {
  something();
}
/* istanbul ignore next -- @preserve */ 
catch(e) {
  // should never happen
}

[Boshen]

#10338 addressed most of the problems, except for the case

  if (arg === 1) {
    return 1;
  } /* istanbul ignore next */ else if (arg === 2) {
    return 2;
  } else {
    return 3;
  }

There is no comment attachment for the else keyword, but we can make it print when the comment is attached to the if statement:

  if (arg === 1) {
    return 1;
  } else /* istanbul ignore next -- @preserve */ if (arg === 2) {
    return 2;
  } else {
    return 3;
  }

I'm liberal around adding these special cases to the codegen, as long as there's a span that the comment can attach to.