`struct`'s modifiers are too sensitive to order

[danzat]

For example, the following generates a parse error (ref is not adjacent to struct):

ref partial struct S {}

Error:

compilation_unit [0, 0] - [2, 0]
  ERROR [0, 0] - [0, 20]
    variable_declaration [0, 0] - [0, 18]
      type: ref_type [0, 0] - [0, 11]
        type: identifier [0, 4] - [0, 11]
      variable_declarator [0, 12] - [0, 18]
        name: identifier [0, 12] - [0, 18]
  global_statement [0, 21] - [0, 23]
    block [0, 21] - [0, 23]

Whereas (ref is adjacent to struct):

partial ref struct S {}

Is fine:

compilation_unit [0, 0] - [1, 0]
  struct_declaration [0, 0] - [0, 23]
    modifier [0, 0] - [0, 7]
    name: identifier [0, 19] - [0, 20]
    body: declaration_list [0, 21] - [0, 23]

It's enough to have just one such error in a file to completely mess up its parsing.