This gets us a few things:
1. Intern strings without wastefully allocating them first in the common fast paths.
1. Switch to a string collection that retains only weak references.

Also revert the Options changes. If the app needs to customize string interning, they can do so by changing the resolver to their own formatter.

This caused a test failure on `net472`.

It is still available via the `StringInterningFormatter`, which can be turned on by default by aggregating a custom resolver,
or by specifying this on a `[Key]` or `[DataMember]` member:

```cs
[MessagePackFormatter(typeof(StringInterningFormatter))]
```

In AOT-only environments it seems it was impossible to avoid loading `StandardResolver` because the `MessagePackSerializer` had a static property with an initializer that loaded the `StandardResolver`.
After this change, if the `DefaultOptions` getter is never called, or its setter is called first, then the `StandardResolver` can be avoided.

Fixes #832

Fixed typo in README.MD

Allows client code to avoid tripping over StandardResolver

Add string interning option

Merge master into develop

Add CompressionMinLength field to unpublished api spec

Before, it only applied to the `Lz4Block` scheme, but I don't see why we would want to apply `Lz4BlockArray` without regard to minimum length, so I've changed the library to always honor the new setting.

Merge master to develop

* Update versions of MPC dependencies
* Add net6.0 target

Co-authored-by: Andrew Arnott <andrewarnott@live.com>

Promote Unshipped APIs to Shipped

Global Using and File Scoped Namespace Test

* Fix nullable member file name handling

Generating multiple files including the letter '?'.
Invalid file name char should be avoided.

update package.json to 2.3.85

Update SDK to 6.0.300

Add built-in support for .NET 6 `DateOnly` and `TimeOnly` types

Fixes #1433

Suppress CS1591 warnings in generated code

Merge master to develop

Where we already had a .NET 6 target, we just drop net5.0. Where .NET 5 was the latest target, we change it to target .NET 6.

This is because .NET 5 is no longer supported by Microsoft, so no one is expected to be using that runtime at this point.

MessagePack.Generator RollForward to Major

Drop net5.0 runtime targets

Bumps [Newtonsoft.Json](https://github.com/JamesNK/Newtonsoft.Json) from 10.0.3 to 13.0.1.
- [Release notes](https://github.com/JamesNK/Newtonsoft.Json/releases)
- [Commits](JamesNK/Newtonsoft.Json@10.0.3...13.0.1)

---
updated-dependencies:
- dependency-name: Newtonsoft.Json
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Bumps [Newtonsoft.Json](https://github.com/JamesNK/Newtonsoft.Json) from 12.0.3 to 13.0.1.
- [Release notes](https://github.com/JamesNK/Newtonsoft.Json/releases)
- [Commits](JamesNK/Newtonsoft.Json@12.0.3...13.0.1)

---
updated-dependencies:
- dependency-name: Newtonsoft.Json
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

…lizerBenchmark/Newtonsoft.Json-13.0.1

Bump Newtonsoft.Json from 12.0.3 to 13.0.1 in /benchmark/SerializerBenchmark

…/Newtonsoft.Json-13.0.1

Bump Newtonsoft.Json from 10.0.3 to 13.0.1 in /sandbox/Sandbox

Allocating 1MB buffers contribute to the large object heap and should be avoidable for applications that it is a problem for.

Add StringInterningFormatter.cs.meta for Unity

Add option to avoid large buffer allocations