Split interceptors into a separate package

[andrewlock]

The interceptor support adds design-time overhead, due to the use of CreateSyntaxProvider that I can't find a good way to avoid. Rather than penalise the base package for an experimental and niche feature, move the interceptor to a separate package.

[github-actions]

Test Results

   143 files   -  11     143 suites   - 11   6m 58s ⏱️ - 1m 59s
 4 095 tests  -  29   4 091 ✅  - 13   4 💤  -  16  0 ❌ ±0 
67 376 runs   - 143  67 310 ✅ +11  66 💤  - 154  0 ❌ ±0 

Results for commit 71eae22. ± Comparison against base commit 651db1b.

This pull request removes 56 and adds 27 tests. Note that renamed tests count towards both.

NetEscapades.EnumGenerators.Interceptors.IntegrationTests.InterceptorTests ‑ CallingToStringIsNotInterceptedWhenNotSupportedBySdk
NetEscapades.EnumGenerators.NetStandard.Interceptors.IntegrationTests.InterceptorTests ‑ CallingToStringIsNotInterceptedWhenNotSupportedBySdk
NetEscapades.EnumGenerators.Nuget.Interceptors.IntegrationTests.InterceptorTests ‑ CallingToStringIsNotInterceptedWhenNotSupportedBySdk
NetEscapades.EnumGenerators.Nuget.Interceptors.IntegrationTests.Roslyn4_4.InterceptorTests ‑ CallingHasFlagIsIntercepted
NetEscapades.EnumGenerators.Nuget.Interceptors.IntegrationTests.Roslyn4_4.InterceptorTests ‑ CallingHasFlagIsIntercepted_ExternalEnumFlags
NetEscapades.EnumGenerators.Nuget.Interceptors.IntegrationTests.Roslyn4_4.InterceptorTests ‑ CallingNonInterceptableEnumIsNotIntercepted
NetEscapades.EnumGenerators.Nuget.Interceptors.IntegrationTests.Roslyn4_4.InterceptorTests ‑ CallingToStringIsIntercepted
NetEscapades.EnumGenerators.Nuget.Interceptors.IntegrationTests.Roslyn4_4.InterceptorTests ‑ CallingToStringIsIntercepted_EnumInFoo
NetEscapades.EnumGenerators.Nuget.Interceptors.IntegrationTests.Roslyn4_4.InterceptorTests ‑ CallingToStringIsIntercepted_EnumWithExtensionInOtherNamespace
NetEscapades.EnumGenerators.Nuget.Interceptors.IntegrationTests.Roslyn4_4.InterceptorTests ‑ CallingToStringIsIntercepted_ExternalEnum
…

NetEscapades.EnumGenerators.Tests.EnumGeneratorInterceptorTests ‑ CanGenerateEnumExtensionsForFlagsEnum(usings: "", attribute: "System.Flags")
NetEscapades.EnumGenerators.Tests.EnumGeneratorInterceptorTests ‑ CanGenerateEnumExtensionsForFlagsEnum(usings: "", attribute: "System.FlagsAttribute")
NetEscapades.EnumGenerators.Tests.EnumGeneratorInterceptorTests ‑ CanGenerateEnumExtensionsForFlagsEnum(usings: "using System;", attribute: "Flags")
NetEscapades.EnumGenerators.Tests.EnumGeneratorInterceptorTests ‑ CanGenerateEnumExtensionsForFlagsEnum(usings: "using System;", attribute: "FlagsAttribute")
NetEscapades.EnumGenerators.Tests.EnumGeneratorInterceptorTests ‑ CanGenerateEnumExtensionsInChildNamespace
NetEscapades.EnumGenerators.Tests.EnumGeneratorInterceptorTests ‑ CanGenerateEnumExtensionsInGlobalNamespace
NetEscapades.EnumGenerators.Tests.EnumGeneratorInterceptorTests ‑ CanGenerateEnumExtensionsInNestedClass
NetEscapades.EnumGenerators.Tests.EnumGeneratorInterceptorTests ‑ CanGenerateEnumExtensionsWithCustomName
NetEscapades.EnumGenerators.Tests.EnumGeneratorInterceptorTests ‑ CanGenerateEnumExtensionsWithCustomNamespace
NetEscapades.EnumGenerators.Tests.EnumGeneratorInterceptorTests ‑ CanGenerateEnumExtensionsWithCustomNamespaceAndName
…