use consistent phrasing for comparing string values

[michaelficarra]

@gibson042 asks what our conventions should be for comparing strings in https://github.com/tc39/ecma262/pull/2721/files/#r890585298.

I think "equals" is indeed bad, but we have many options available:

• SameValue
• "is the same string value"
• "is the same sequence of code units"
• SameValueNonNumeric
• "is"

Let's pick something and make it consistent.

[bakkot]

I like "is the same string value" personally. We could dfn the term if we're worried it might not be clear.

[michaelficarra]

We could also use the phrase within SameValueNonNumeric and use what is currently there in the dfn for that phrase.

[michaelficarra]

Also, "is the empty String" should remain.

[michaelficarra]

Related: #2405

Since strings don't carry identity, "is" should be fine. Also because of this, we can do things like if _list_ contains _string_ instead of something weird like if _list_ contains a string with the same string value as _string_.

[michaelficarra]

Discussed at the editor call today. "is" or "=" would be preferable as long as we've made it clear that strings do not carry identity, so I'm going to try to fix #2405 first. We could probably then use this phrasing for all values which are declared not to have identity.