Add support for schema extensions#16

When running the formatter against schemas with extension, the following error is displayed: ``` ***/app/node_modules/format-graphql/dist/utilities/formatSdl.js:30 return a.name.value.localeCompare(b.name.value); ^ TypeError: Cannot read properties of undefined (reading 'value') at ***/app/node_modules/format-graphql/dist/utilities/formatSdl.js:30:48 at Array.sort (<anonymous>) at sortSchema (***/app/node_modules/format-graphql/dist/utilities/formatSdl.js:20:26) at walkAST (***/app/node_modules/format-graphql/dist/utilities/formatSdl.js:83:15) at formatSdl (***/app/node_modules/format-graphql/dist/utilities/formatSdl.js:90:30) at Object.<anonymous> (***/app/node_modules/format-graphql/dist/bin/index.js:52:44) at Module._compile (node:internal/modules/cjs/loader:1101:14) at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10) at Module.load (node:internal/modules/cjs/loader:981:32) at Function.Module._load (node:internal/modules/cjs/loader:822:12) error Command failed with exit code 1. ``` Example schema: ``` extend schema @link(url: "https://specs.apollo.dev/federation/v2.0", import: ["@key", "@shareable"]) type GenericResponse @shareable { message: String success: Boolean! } ```