[video_player_web] Adjust Web implementation to the new platform interface

[FirentisTFW]

This PR adjusts the Web video_player implementation to the new platform interface which was introduced in #8453. Specifically, it renames textureId to playerId (which actually makes sense here, as there are no texture IDs on Web, it was just a "simulation" to comply with the interface).

This PR does not fix any issue directly, but it's related to #86613.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
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• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[stuartmorgan]

LGTM, thanks for the quick cleanup!

[FirentisTFW]

@ditman Would you like to do the secondary review?

[stuartmorgan]

@FirentisTFW Now that you have the magic Member badge, you no longer need a secondary review; feel free to land it :)

(If this changed non-trivial web logic I'd definitely want someone with more web expertise than me to review it, but this is a straightforward PR.)