[C# 14-Errors and warnings]: New Feature - `extension` as an identifier

[BillWagner]

Visual Studio release

other - specify below

Link to speclet

• Speclet

Link to Pull request

• PR

Errors and warnings added for this feature

• "CS9306"

This is better combined with other errors related to identifier names. So, I split this from #46376

What language theme applies, for consolidation of articles

No response

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Associated WorkItem - 436519