Support JSX fragments

Steps to reproduce

esprima.parse('<></>', { jsx: true })

Expected output

// excludes location information for brevity
{
  "type": "Program",
  "body": [
    {
      "type": "ExpressionStatement",
      "expression": {
        "type": "JSXFragment",
        "openingFragment": {
          "type": "JSXOpeningFragment"
        },
        "closingFragment": {
          "type": "JSXClosingFragment"
        }
      }
    }
  ],
  "sourceType": "module"
}

Actual output

This exception is thrown: Line 1: Unexpected token >

Relevant references

Looks like jsx-parser.ts would have to change here, and possibly elsewhere. This syntax works fine in @babel/parser and acorn.

Start with the JSX parsing logic in src/jsx-parser.ts at the referenced location, and reproduce the issue using esprima.parse('<></>', { jsx: true }). Compare the result with the expected JSXFragment AST in the issue. Done means this input parses successfully and produces the shown openingFragment and closingFragment nodes.