Upgrade extension API

This release introduces many major changes to public API and breaks BC for the sake of more solid and stable API in a further versions.

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Changes:

• Drop PHP 7.0 support, #30;
• Add typehint for $value param in Template::set(), #36;
• Rename constants to match PSR-2 convention, #34;
• Make enum classes with constants final, #39 ;
• Rename HeapStatistics methods to match PSR-2, #38;
• Rename V8\Exception class and it methods, #33;
• Add FunctionObject::getScriptId() method;
• Remove no-data constants and use NULL instead, #40;
• Refactor ScriptOriginOptions to use bit field, #42;
• Remove redundant ScriptOrigin::is*() methods, #61;
• Add missed method to Value stub;
• Fix improperly adjusted external allocated memory on weak callback call;
• Add Proxy and Promise builtin support, #55;
• Add support for regexp dotAll flag, #54;
• Move RegExpObject\Flags constants under RegExpObject, #64;
• Move CompileOptions constants under ScriptCompiler, #65;
• Add Isolate::MemoryPressureNotification() support, #57;
• Require libv8 >= 6.3.163;
• Add StackFrame::isWasm(), #53;
• Get rid of CallbackInfo, #67;
• Add immutable prototype chains to ObjectTemplate, #58;
• Add StartupData::warmUpSnapshotDataBlob(), #59;
• Add missed JSON class support, #56;
• Add Message::getErrorLevel() method, #52;
• Rename SymbolValue methods, #41;
• Rename TryCatch methods, #41;
• Add missed PrimitiveValue::value() abstract method;
• Add Isolate::SetRAILMode(), #60;
• Fix improperly internally constructed RAILMode enum class, #60.

See #43 (comment) for details