feat: Add DynamicMap type

[mattjohnsonpint]

Description

This adds a DynamicMap type to the Modus AssemblyScript SDK. It allows a user (or another sdk function) to have the equivalent of Map<string, any> in AssemblyScript, with some caveats:

• Only string is supported for the map key
• Any value must be JSON serializable, either by being a primitive type or being decorated with @json
• Iterating map values with .values() will return the raw JSON value for each item, since it's not possible to infer the type.

Note, the bug described in JairusSW/as-json#98 required some extensive workarounds in the implementation that may be buggy, but have passed the basic tests. We'll want to remove the workarounds when the bug is fixed upstream.

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to this PR
• Tests added for new functionality, or regression tests for bug fixes added as applicable