feature: Alternative handling of trailing backslashes in macro definitions

Did you check the tree-sitter docs?

• I have read all the tree-sitter docs if it relates to using the parser

Is your feature request related to a problem? Please describe.

Currently, when defining a macro with a trailing backslash \ at the end of the last line, the parse tree extends the preproc_arg end position to the next line.

For example, given:

#define ADD(x, y) \
    ((x) + (y)) \

The parse tree is:

translation_unit [0, 0] - [2, 0]
  preproc_function_def [0, 0] - [2, 0]
    #define [0, 0] - [0, 7]
    name: identifier [0, 8] - [0, 11]
    parameters: preproc_params [0, 11] - [0, 17]
      ( [0, 11] - [0, 12]
      identifier [0, 12] - [0, 13]
      , [0, 13] - [0, 14]
      identifier [0, 15] - [0, 16]
      ) [0, 16] - [0, 17]
    value: preproc_arg [1, 4] - [2, 0]    // Ends at position [2, 0]

Describe the solution you'd like

I was instead wondering if it would be possible to ensure that, in the parse tree, even with a \ at the end of the last line of the macro, the final position of the preproc_arg node matches the following case:

#define ADD(x, y) \
    ((x) + (y))

Where the resulting parse tree is:

translation_unit [0, 0] - [2, 0]
  preproc_function_def [0, 0] - [2, 0]
    #define [0, 0] - [0, 7]
    name: identifier [0, 8] - [0, 11]
    parameters: preproc_params [0, 11] - [0, 17]
      ( [0, 11] - [0, 12]
      identifier [0, 12] - [0, 13]
      , [0, 13] - [0, 14]
      identifier [0, 15] - [0, 16]
      ) [0, 16] - [0, 17]
    value: preproc_arg [1, 4] - [1, 16]    // Ends at position [1, 16]

The issue names no source file or test. Start with the two C++ macro examples and compare their parse-tree endpoints, then read the linked tree-sitter parser documentation and trace how the preproc_arg node is produced. Done means the trailing-backslash case ends at [1, 16], matching the no-trailing-backslash example.