fix(embed): escape dollar symbol of JSON string literal

[Jumpaku]

With the original version, embed.dart cannot generate a compilable Dart code if the source JSON has a string literal containing a dollar symbol ($). For example, the following code will be generated from {"dollar": "$"}:

const _$variable = (dollar: "$");
// the string literal "$" is not valid, which is a beginning of string interpolation.

In the revised version, I fixed it to generate the following code from that JSON:

const _$variable = (dollar: "\$");
// $ is escaped!

I also added a test case.

[fujidaiti]

Hi @Jumpaku,

Nice work, thank you!