[svelte-parse] Modifiers on attributes without values

[halfnelson]

when the parser encounters an attribute with a modifier but no value, it continues to add tokens to the modifier name until >or =

<a on:click|preventDefault booleanAttribute></a>

gives

      {
          "type": "svelteDirective",
          "name": "on",
          "value": [],
          "modifiers": [
            {
              "value": "preventDefaultbooleanAttribute",
              "type": "modifier",
              "position": {
                "start": {
                  "line": 1,
                  "column": 12,
                  "offset": 11
                },
                "end": {}
              }
            }
          ],
          "shorthand": "none",
          "position": {
            "start": {
              "line": 1,
              "column": 4,
              "offset": 3
            },
            "end": {}
          },
          "specifier": "click"
        }