Describe the bug
It looks like react-pdf should support font weights (#535), but this does not seem to be the case for the included fonts.

To Reproduce
The snippet below creates three lines. The first is normal weight for reference, and the other two are different ways of setting font weight; The first uses the fontWeight modifier, the other explicitly references Helvetica-Bold. The second and third should look the same, since { fontFamily: 'Helvetica', fontWeight: 'bold' } should be equivalent to { fontFamily: 'Helvetica-Bold' }

REPL with the code below

const doc = (
  <Document>
    <Page size="A4">
      <View style={{ font: 'Helvetica', fontWeight: 'normal' }}>
        <Text>Section #1 (normal text)</Text>
        <Text style={{ fontFamily: 'Helvetica', fontWeight: 'bold' }}>Section #1 (fontWeight only)</Text>
        <Text style={{ fontFamily: 'Helvetica-Bold' }}>Section #1 (explicit fontFamily)</Text>      
      </View>
    </Page>
  </Document>
);

ReactPDF.render(doc);

Expected behavior
{ fontFamily: 'Helvetica', fontWeight: 'bold' }
should be equivalent to
{ fontFamily: 'Helvetica-Bold' }

Desktop (please complete the following information):

• MacOS
• chrome
• React-pdf version v1.6.0 in the REPL, v1.6.2 locally