Escape not just backticks but ${ as well in graphql tags

[lydell]

Fixes #4974.

• I’ve added tests to confirm my change works.
• (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory)
• I’ve read the contributing guidelines.

[lydell]

The original escaping here is lost, but that’s another issue.

[lydell]

Still seems to fail somehow :(

Prettier pr-5137
Playground link

--parser babylon

Input:

gql`
query {
a(c: "a \\ b ") {
b
}
}
`

Output:

SyntaxError: Syntax Error: Invalid character escape sequence: \ . (3:10)
  1 | 
  2 | query {
> 3 | a(c: "a \ b ") {
    |          ^
  4 | b
  5 | }
  6 | }

EDIT: No, I'm just confused. The above is supposed to fail. Here's what I meant:

Prettier pr-5137
Playground link

--parser babylon

Input:

gql`
query {
a(c: "a \\\\ b ") {
b
}
}
`

Output:

gql`
  query {
    a(c: "a \\\\ b ") {
      b
    }
  }
`;

I'm starting to hate escaping.