Add role=image as synonym for role=img - #1370

Co-authored-by: James Craig <cookiecrook@users.noreply.github.com>

Add a reusable synonym mechanism: the image role is defined as a synonym
of img (ARIA 1.3, w3c/aria#1370) and getExplicitRole/getRole resolve
synonym roles to their canonical role by default, with a noSynonym opt
out. The role-img-alt selector and aria-allowed-role role resolution are
updated so role=image behaves like role=img.

Closes #4656

Add the ARIA 1.3 image role (w3c/aria#1370) as an alias of img: it is a
valid role sharing img's definition (spread from a common object) and is
allowed wherever img is allowed in the html-elms spec (embed, iframe,
svg). The role-img-alt selector matches both roles via :is().

This avoids a synonym-resolution mechanism, so the role the author used
is preserved when reporting an unallowed role.

Closes #4656

Adds support for `role="image"` as a valid role equivalent to
`role="img"` (ARIA 1.3,
[w3c/aria#1370](w3c/aria#1370)).

## What & why

ARIA 1.3 adds `image` as an alias of `img` with an identical mapping;
axe previously treated `role="image"` as an invalid role.

Per the review discussion, this does **not** use a synonym-normalization
mechanism (which would force every reporting path to remember the role
the author used vs. the one our code resolved it to). Instead, `image`
is a first-class role that shares `img`'s definition and is allowed
wherever `img` is:

- `image` is added to `lib/standards/aria-roles.js`, spread from a
shared `imgRole` object so it stays in sync with `img`.
- `image` is added to every `html-elms` `allowedRoles` list that
includes `img` (`embed`, `iframe`, `object`). `svg` needs no change — it
is `allowedRoles: true`, so it already accepts any role.
- The `role-img-alt` and `svg-img-alt` selectors match both roles via
`:is([role='img'], [role='image'])`. Without the `svg-img-alt` change,
`<svg role="image">` matched neither rule — `svg-img-alt`'s selector
omitted it and `role-img-alt` is gated by `html-namespace-matches` — so
axe accepted the role and then never checked for an accessible name.
- `image` is added to the `aria-roledescription` check's
`supportedRoles`, so `role="image"` passes where `role="img"` does.

Because there is no synonym resolution, an unallowed `role="image"`
reports **`image`** (the role the author used), not `img`.

## Tests

- `is-valid-role` / `aria-roles` integration — `image` is a valid role.
- `get-element-unallowed-roles` — `image` allowed on
`svg`/`embed`/`object`/`iframe`; reported as authored when unallowed
(e.g. on `hr`).
- `role-img-alt` integration + virtual-rules — matches `role="image"`.
- `svg-img-alt` integration + virtual-rules — matches `role="image"`.
- `aria-roledescription` — `role="image"` passes against the check's
default `supportedRoles`.
- ACT specs `7d6734` and `23a2a8` still pass with the broadened
selectors.

## Not in scope

`role-img-alt` and `svg-img-alt` metadata still name only `img` in their
`description`/`help` strings. Updating them regenerates
`locales/_template.json`, invalidates those translations, and needs a
matching axe-rule-help change — tracked in #5272.

Closes #4656

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Co-authored-by: Steven Lambert <2433219+straker@users.noreply.github.com>