Implement ISpanAppendable in CharBlockArray and CharTermAttributeImpl

[paulirwin]

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Implement ISpanAppendable in CharBlockArray and CharTermAttributeImpl

Fixes #1026

Description

This adds ISpanAppendable to the CharBlockArray and CharTermAttributeImpl classes. The classes implement the interface explicitly so that we can provide a default implementation that returns itself rather than ISpanAppendable, to match the existing Append overloads.

Note that while IAppendable is now redundant in the interface list for these types, I left it there for Lucene source compatibility and reference, since ISpanAppendable does not exist in Lucene.

[paulirwin]

@NightOwl888 I intend to mark this ready for review once #1018 is merged, leaving as draft for now. I'll also likely have to resolve a merge conflict at that point too.

[NightOwl888]

Thanks for the PR.

There were a few more changes included than required. Copying spans is much less optimized on .NET Framework than using other copy methods, so we should add the Append(ReadOnlySpan<char>) method without changing the implementations of the other overloads. Please see the inline comments.

All requested changes implemented