[unused-fields] Add option to check leave fields only

[janicduplessis]

Builds on top of #94 for option parsing code. Check commit 8d0773f only.

Motivation

This makes it so we only check leave fields, I'm not sure if there is actually cases where checking non-leaves is useful, so I left it as an option. If there are not we could make this the default. There are cases where non leave fields are never accessed because of helper functions in other files.

For example I use a useConnectionArray hook to transform a connection to an array. This will currently trigger the unused field rule since the current file never accesses edges and node.

Example:

graphql`fragment TestNested on Page { connection { edges { node { id } } } }`;
...
const nodes = useConnectionArray(props.connection);
...
nodes[0].id

// error: unused edges, node

Test plan

Tested on a large relay codebase and added tests.

[jscheid]

+1, we have the same problem. We'd like to use a helper to extract nodes from a connection and it generates false positives. The reasoning seems sound - if leaves are being used then the intermediary nodes are being used implicitly, even if the code can't detect it reliably. It would be great if this could get merged.

[sibelius]

very common pattern

[AugustoCalaca]

does it works well if we use options, for some reason, as [{leavesOnly: true}, ignoreFields: ['unsed2']]?
I think that can add a new test

{
  code: `
    graphql\`fragment Test on Page { unused1 { unused2 } }\`;
  `,
  options: [{leavesOnly: true}, ignoreFields: ['unsed2']],
},