Add Symbol.Parse, make Parser static

[adamsitnik]

• expose ParseResult Symbol.Parse(string[] args, CommandLineConfiguration = default), so users can easily discover parsing
• make Parser a static type with a single ParseResult Parse(string[] args, CommandLineConfiguration = default) method
• make CommandLineBuilder.Build return CommandLineConfiguration, move all extension methods from CommandLineBuilderExtensions to CommandLineBuilder itself
• Move the actual parsing logic from StringExtensions to Parser:

https://github.com/dotnet/command-line-api/blob/main/src/System.CommandLine/Parsing/StringExtensions.cs#L72