Add extension members for static extensions in generated enums

[andrewlock]

Fixes #136

[github-actions]

Test Results

    162 files  ±     0      162 suites  ±0   2m 8s ⏱️ +13s
  7 426 tests +   689    7 422 ✅ +   689   4 💤 ±0  0 ❌ ±0 
129 130 runs  +12 005  129 058 ✅ +12 005  72 💤 ±0  0 ❌ ±0 

Results for commit b8a6508. ± Comparison against base commit 8439485.

This pull request removes 3 and adds 692 tests. Note that renamed tests count towards both.

NetEscapades.EnumGenerators.Tests.SourceGenerationHelperSnapshotTests ‑ GeneratesEnumCorrectly
NetEscapades.EnumGenerators.Tests.SourceGenerationHelperSnapshotTests ‑ GeneratesEnumWithRepeatedValuesCorrectly
NetEscapades.EnumGenerators.Tests.SourceGenerationHelperSnapshotTests ‑ GeneratesFlagsEnumCorrectly

NetEscapades.EnumGenerators.IntegrationTests.EnumInFooExtensionEverythingTests ‑ GeneratesAsUnderlyingType(value: 3)
NetEscapades.EnumGenerators.IntegrationTests.EnumInFooExtensionEverythingTests ‑ GeneratesAsUnderlyingType(value: First)
NetEscapades.EnumGenerators.IntegrationTests.EnumInFooExtensionEverythingTests ‑ GeneratesAsUnderlyingType(value: Second)
NetEscapades.EnumGenerators.IntegrationTests.EnumInFooExtensionEverythingTests ‑ GeneratesGetNames
NetEscapades.EnumGenerators.IntegrationTests.EnumInFooExtensionEverythingTests ‑ GeneratesGetValues
NetEscapades.EnumGenerators.IntegrationTests.EnumInFooExtensionEverythingTests ‑ GeneratesGetValuesAsUnderlyingType
NetEscapades.EnumGenerators.IntegrationTests.EnumInFooExtensionEverythingTests ‑ GeneratesIsDefined(value: 3)
NetEscapades.EnumGenerators.IntegrationTests.EnumInFooExtensionEverythingTests ‑ GeneratesIsDefined(value: First)
NetEscapades.EnumGenerators.IntegrationTests.EnumInFooExtensionEverythingTests ‑ GeneratesIsDefined(value: Second)
NetEscapades.EnumGenerators.IntegrationTests.EnumInFooExtensionEverythingTests ‑ GeneratesIsDefinedUsingName(name: "-267")
…