Extensions: resolve some follow-up comments on signature conflicts and betterness

[jcouv]

From discussion in WG, the implementation follows the design that was discussed in LDM for signature conflicts and for betterness, and the implications were understood. This PR is a shout test, in case you find that any of the interesting scenarios should warrant a design change.

Relates to test plan #76130

[AlekseyTs]

    Assert.Equal("System.Int32 E.<>E__0.P { get; }", model.GetSymbolInfo(memberAccess).Symbol.ToTestDisplayString());

It is hard to say which property is used since both extensions are in the same class. Perhaps ToDisplayString will be more user-friendly. #Pending

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Refers to: src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs:28083 in ba65636. [](commit_id = ba65636, deletion_comment = False)

[AlekseyTs]

    Assert.Equal("System.Int32 E.<>E__1.P { get; }", model.GetSymbolInfo(memberAccess).Symbol.ToTestDisplayString());

Similar comment #Pending

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Refers to: src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs:28114 in ba65636. [](commit_id = ba65636, deletion_comment = False)

[AlekseyTs]

LGTM (commit 1)