RSDL Editor doesn't distinguish property named "key" from keyword

[mikepizzo]

Steps to reproduce:

1. Go to https://rapid.rocks/api-designer/index.html and copy the following into the RSDL Editor:

type myType
{
  key: String
}

Result: the property name "key" is highlighted as if it is a keyword. If "key" is followed by a colon it should be treated as a literal (note that CSDL generation does get this right).

[ralfhandl]

Probably part of issue #393