Normative: Allow Annex B scripts to start with -->

[nicolo-ribaudo]

This PR updates the spec to match web reality (tested in Safari, Firefox, and Chrome -- XS does not implement HTML comments).

Annex B allows HTML closing comments (--> foo) after a line terminator, potentially preceded by other comments or spaces (SingleLineHTMLCloseComment). However, browsers also support them at the beginning of scripts. Some test cases:

• eval("--> foo\nconsole.log(123)");

• <script>--> foo
  console.log(123);
  </script>

• <script src="./script.js"></script>

  --> foo
  console.log(123);

• eval("/* comment */ --> foo\nconsole.log(123)");

• (this case is already supported by the current spec)

  eval("/* com\nment */ --> foo\nconsole.log(123)");

This PR accomplishes this by adding HTMLCloseComment to InputElementHashbangOrRegExp in Annex B, where InputElementHashbangOrRegExp is the parsing goal used for the first token in the script.

---

EDIT There is already a PR fixing this, by @leobalter (#1493), but it was stuck on discussing how to specify this. My version has the advantage of not introducing any new spec language, but I'm happy to close this PR if we find the original one more editorially appealing.

Notes from discussing this in the past: https://github.com/tc39/notes/blob/main/meetings/2019-03/mar-26.md#normative-createdynamicfunction-early-concatenates-body

[nicolo-ribaudo]

@gibson042 You mentioned on matrix that

All implementations that I tested reject a script starting with "-->", but JSC and SM and V8 incorrectly fail to reject eval input that does so:

but all the browsers I tested accept scripts starting with -->. Do you have a test case?

[gibson042]

All implementations that I tested reject a script starting with "-->", but JSC and SM and V8 incorrectly fail to reject eval input that does so:

but all the browsers I tested accept scripts starting with -->. Do you have a test case?

It was tested with eshost, which might well be modifying the source in a way that is relevant. I was able to reproduce your acceptance in browsers, node, and esvu-fetched JavaScriptCore, SpiderMonkey, and V8.

[nicolo-ribaudo]

I'll work on tests during this meeting so that it can be merged on time for 2024.

[nicolo-ribaudo]

Tests: tc39/test262#4002