[w3c/manifest] we refer to ASCII lowercase from WHATWG infra spec but Trim from ES spec (#600)

We should be more consistent

ie we could refer to ToLowerCase from ECMA as well

```When instructed to ToLowerCase(x), a user agent must behave as if [ECMASCRIPT]'s String.prototype.toLowerCase() function had been called on the string x.```
      
Or we could refer to infra instead of Trim()
https://infra.spec.whatwg.org/#strip-leading-and-trailing-ascii-whitespace

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