Infer end position for Pylint diagnostics

Pylint only supplies the start position of markers, but not the end.
Always extending the marker until the end of the line leads to
undesirable highlighting in editors, for example:

```python
def my_func(unused_arg, used_arg):
    print(used_arg)
```

Starting an "unused argument" diagnostic at `unused_arg` and extending
It until EOL would also highlight `used_arg`, even though the diagnostic
doesn't apply there.

With this commit, the end position sent to the editor is automatically
determined by looking for an identifier at the indicated start position.
If there is no valid identifier at that position, the entire line is
highlighted as before.

Author: Wuestengecko

pylsp/plugins/pylint_lint.py

@@ -151,7 +151,11 @@ def lint(cls, document, is_saved, flags=""):
                     "line": line,
                     # It's possible that we're linting an empty file. Even an empty
                     # file might fail linting if it isn't named properly.
-                    "character": len(document.lines[line]) if document.lines else 0,
+                    "character": (
+                        _find_end_of_identifier(document.lines[line], diag["column"])
+                        if document.lines
+                        else 0
+                    ),
                 },
             }
 
@@ -338,8 +342,9 @@ def _parse_pylint_stdio_result(document, stdout):
                 "start": {"line": line, "character": character},
                 "end": {
                     "line": line,
-                    # no way to determine the column
-                    "character": len(document.lines[line]) - 1,
+                    "character": _find_end_of_identifier(
+                        document.lines[line], character
+                    ),
                 },
             },
             "message": msg,
@@ -352,3 +357,11 @@ def _parse_pylint_stdio_result(document, stdout):
         diagnostics.append(diagnostic)
 
     return diagnostics
+
+
+def _find_end_of_identifier(string, start):
+    """Find the end of the identifier starting at the given position."""
+    for i in range(len(string), start, -1):
+        if string[start:i].isidentifier():
+            return i
+    return len(string) - 1