Original commit message:

    [import-attributes] Implement import attributes, with `assert` fallback

    In the past six months, the old import assertions proposal has been
    renamed to "import attributes" with the follwing major changes:
    1. the keyword is now `with` instead of `assert`
    2. unknown assertions cause an error rather than being ignored

    To preserve backward compatibility with existing applications that use
    `assert`, implementations _can_ keep it around as a fallback for both
    the static and dynamic forms.

    Additionally, the proposal has some minor changes that came up during
    the stage 3 reviews:
    3. dynamic import first reads all the attributes, and then verifies
       that they are all strings
    4. there is no need for a `[no LineTerminator here]` restriction before
       the `with` keyword
    5. static import syntax allows any `LiteralPropertyName` as attribute
       keys, to align with every other syntax using key-value pairs

    The new syntax is enabled by a new `--harmony-import-attributes` flag,
    disabled by default. However, the new behavioral changes also apply to
    the old syntax that is under the `--harmony-import-assertions` flag.

    This patch does implements (1), (3), (4) and (5). Handling of unknown
    import assertions was not implemented directly in V8, but delegated
    to embedders. As such, it will be implemented separately in d8 and
    Chromium.

    To simplify the review, this patch doesn't migrate usage of the term
    "assertions" to "attributes". There are many variables and internal
    functions that could be easily renamed as soon as this patch landes.
    There is one usage in the public API
    (`ModuleRequest::GetImportAssertions`) that will probably need to be
    aliased and then removed following the same process as for other API
    breaking changes.

    Bug: v8:13856
    Change-Id: I78b167348d898887332c5ca7468bc5d58cd9b1ca
    Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/4632799
    Commit-Queue: Shu-yu Guo <syg@chromium.org>
    Reviewed-by: Adam Klein <adamk@chromium.org>
    Cr-Commit-Position: refs/heads/main@{#89110}

Refs: v8/v8@159c82c
Refs: v8/v8@a0fd320
PR-URL: #50183
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>

Original commit message:

    [import-attributes] Remove support for numeric keys

    During the 2023-09 TC39 meeting the proposal has been updated to remove support
    for bigint and float literals as import attribute keys, due to implementation
    difficulties in other engines and minimal added value for JS developers.

    GH issue: tc39/proposal-import-attributes#145

    Bug: v8:13856
    Change-Id: I0ede2bb10d6ca338a4b0870a1261ccbcd088c16f
    Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/4899760
    Reviewed-by: Shu-yu Guo <syg@chromium.org>
    Commit-Queue: Joyee Cheung <joyee@igalia.com>
    Cr-Commit-Position: refs/heads/main@{#90318}

Refs: v8/v8@ea996ad
PR-URL: #50183
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>