Supports for constant defined keys

[merschformann]

Description

Support map keys defined via const instead of string literals, and expand test coverage across all supported styles.

Changes

• Switch load mode to LoadModeTypesInfo so the analyzer can resolve constant values via pass.TypesInfo
• When a key is an identifier, resolve it to its const string value and check that instead of checking the identifier name
  • Non-constant (var) identifiers are silently skipped since their value is not statically knowable
• Improve test coverage and fix some tests.