Add ConfigureAwaitOptions polyfill for pre-net8.0

[paulomorgado]

Closes #527

Add ConfigureAwaitOptions enum and Task.ConfigureAwait(ConfigureAwaitOptions) / Task<TResult>.ConfigureAwait(ConfigureAwaitOptions) extension methods for frameworks prior to .NET 8.0.

What's included

• ConfigureAwaitOptions enum — [Flags] enum with None, ContinueOnCapturedContext, SuppressThrowing, and ForceYielding values. Guarded with #if !NET8_0_OR_GREATER and includes TypeForwardedTo for net8.0+. Supports PolyPublic and PolyUseEmbeddedAttribute.
• Task.ConfigureAwait(ConfigureAwaitOptions) — Extension method supporting all options. ForceYielding is implemented via Task.Yield() wrapper; SuppressThrowing via try/catch wrapper.
• Task<TResult>.ConfigureAwait(ConfigureAwaitOptions) — Extension method that rejects SuppressThrowing with ArgumentOutOfRangeException (matches BCL behavior — can't suppress and return default TResult).
• Invalid flags throw ArgumentOutOfRangeException for both Task and Task<TResult>.
• Restructured Polyfill_Task.cs from file-level #if to inner guards to accommodate both #if !NET6_0_OR_GREATER (WaitAsync) and #if !NET8_0_OR_GREATER (ConfigureAwait) in the same file.
• 13 tests covering all options, combinations, error cases, and ForceYielding behavioral assertions (verifies GetAwaiter().IsCompleted returns false for completed tasks).
• Consume usage and Split files for all 18 pre-net8.0 framework directories.

Notes

• Split files were manually generated (ApiBuilderTests requires .NET 11 SDK). They should be regenerated when .NET 11 SDK is available.
• Tests pass on net8.0 (1317 tests) and net10.0 (1320 tests).

[SimonCropp]

C:\projects\polyfill\src\Polyfill\Polyfill_Task.cs(107,1): error CS1519: Invalid token '}' in a member declaration [C:\projects\polyfill\src\Polyfill\Polyfill.csproj]

[SimonCropp]

actually this lgtm. i will fix the problem after merge