- relates to #89 because that changes `string` property checks and needs this refactor
- determine `string`-ness when creating `TagHelperAttributeDescriptor`s
- add `TagHelperAttributeDescriptor.IsStringProperty` (set in constructor)
- avoid repeated `string` comparisons and be more explicit
- change `TagHelperBlockRewriter` to centralize more of the `string`-ness determination
- also add `TryParseResult` DTO, avoiding multiple `out` parameters
- refactor `CSharpTagHelperCodeRenderer` to allow reuse of core attribute value rendering
- test all of it
- add `TagHelperDescriptorTest` to confirm serialization / deserialization
minor:
- fix `TagHelperBlockRewriter.TryParseBlock()` end quote removal when tag is malformed
nits:
- remove dangling mention of fixed bug #220
- make recently-added `TagHelperBlockRewriterTest` tests realistic
- multiple `TagHelperDescriptor`s for same tag helper have identical `Attributes`
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