Support define symbols in .asmdef files

[citizenmatt]

This PR will:

• Parse and cache define symbols from .asmdef files, evaluating rules against installed packages
• Provide context to the parser for source code from compiled packages, so that the correct #if branches are highlighted (RIDER-50221 + Wrong define symbols in packages #710)
• Grey out defineConstraints values that are not defined for a .asmdef file
• Grey out versionDefines symbols that are not defined because a package is not installed or does not meet the version expression
• Show version of installed packages as an inlay hint (with options) for the name property of versionDefines
• Provide code completion for symbols in defineConstraints
• Provide code completion for packages and the special "Unity" symbol in the name property for versionDefines
• Validation for symbols in defineConstraints and versionDefines.define and the version range versionDefines.expression

Also introduces nullable reference types in modified files, because we're on C#9, might as well use it.