Add mergeAST utility for merging multiple AST documents#4582

veeceey wants to merge 2 commits intographql:16.x.xgraphql/graphql-js:16.x.xfrom

veeceey:fix/issue-1428-merge-ast-treesveeceey/graphql-js:fix/issue-1428-merge-ast-treesCopy head branch name to clipboard