[import-attributes] Rename "assertions" to "attributes" in internal code

This patch renames import occurrences of import assertions in internal
code. Regardless of what will happen to the old keyword, these
"parameters" are now called assertions and using the new name will make
it easier for who in a few years will read the code without being aware
of the history of the proposal.

Variables representing the second parameter of a dynamic import call
have been renamed to "import options" rather than "import attributes",
since the attributes are the `with`/`assert` property of that object.

This patch still leaves the old name in public code:
- V8 API:
  - `ModuleRequest::GetImportAssertions`
  - `HostImportModuleDynamicallyWithImportAssertionsCallback`
- Error messages

Bug: v8:13856
Change-Id: Ia2ea13adbfdb65c0a724ef0eab38d1057440e6c2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/5191569
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Nicolò Ribaudo <nribaudo@igalia.com>
Cr-Commit-Position: refs/heads/main@{#92034}