Update highlighting for synopsis metadata in comments

[BernieWhite]

Add grammar highlighting to support use of Synopsis: similar to Description: used in comment based metadata.

For example:

# Synopsis: Redis Cache should reject TLS versions older then 1.2
Rule 'redis.MinTLS' -If { ResourceType 'Microsoft.Cache/Redis' } {
    # Check that TLS is within range
    Within 'properties.minimumTlsVersion' '1.2'
}

This is related to BernieWhite/PSRule#164

Use of Description: is deprecated and will be removed in the future, but right now both are supported and should be highlighted correctly.