Add highlighting for double-quotes delimited tokens and string literals

Role of tokens wrapped with double quotes is mentioned in https://boostorg.github.io/build/manual/develop/index.html#jam.language.lexical

How are the double quotes different in 

```
local s = "abc";
```

versus

```
<toolset>gcc:<cxxflags>"-std=c++11 -pedantic"
```

Are both string literals?