robust when deal big fortran project

[foranewlife]

Is your feature request related to a problem? Please describe. Recently, I use vscode with fortls and i find a bug. When fortran project is big(like cp2k), fortls/langserver.py:1473 will use up all thread resources, as #207 shows. and parsers/internal/parser.py:2257 will show this error

  File "/home/chenys/miniforge3/lib/python3.10/site-packages/fortls/parsers/internal/parser.py", line 2257, in preprocess_file
    for include_dir in include_dirs:
RuntimeError: Set changed size during iteration

Describe the solution you'd like I fix this issue with those code.

1. use concurrent.futures to fix using up threads pool fortls/langserver.py:1473:

   
   from concurrent.futures import ThreadPoolExecutor, as_completed
   def workspace_init(self):
       """Initialize the workspace root across multiple threads"""
   
       file_list = self._get_source_files()
       results = {}
   
       # Use ThreadPoolExecutor for better management
       with ThreadPoolExecutor(max_workers=self.nthreads) as executor:
           future_to_filepath = {
               executor.submit(
                   self.file_init,
                   filepath,
                   self.pp_defs,
                   self.pp_suffixes,
                   self.include_dirs,
                   self.sort_keywords
               ): filepath for filepath in file_list
           }
   
           for future in as_completed(future_to_filepath):
               filepath = future_to_filepath[future]
               try:
                   result_obj = future.result()
               except Exception as exc:
                   self.post_message(
                       f"Initialization failed for file {filepath}: {exc}"
                   )
                   continue
   
               if isinstance(result_obj, str):
                   self.post_message(
                       f"Initialization failed for file {filepath}: {result_obj}"
                   )
                   continue
   
               self.workspace[filepath] = result_obj
               ast_new = self.workspace[filepath].ast
               for key in ast_new.global_dict:
                   self.obj_tree[key] = [ast_new.global_dict[key], filepath]
   
       # Update include statements
       for file_obj in self.workspace.values():
           file_obj.ast.resolve_includes(self.workspace)
   
       # Update inheritance/links
       self.link_version = (self.link_version + 1) % 1000
       for file_obj in self.workspace.values():
           file_obj.ast.resolve_links(self.obj_tree, self.link_version)

2. copy a new variable to fix error.
`parsers/internal/parser.py:2257`:
```python
            include_dirs_copy = include_dirs.copy()

            for include_dir in include_dirs_copy:

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Additional context Add any other context or screenshots about the feature request here.