refactor(backends): remove singledispatchmethod from the sql backends (…

…ibis-project#8338)

## Description of changes

This pull request does two things:

1. Removes use of `singledispatchmethod` in the SQL compilers
1. Fixes the support matrix accuracy for SQL backends

Follow-ups:

* ~Deal with filtering out RisingWave geospatial~ Handled here
* `__init_subclass__` for `SIMPLE_OPS` handled here
* Fix coverage accuracy for the non-SQL backends

## Issues closed

Fixes ibis-project#8283.

Thanks to @jcrist for the `__init_subclass__` tip, that saved N backends
duplication of filling in undefined operations.

docs/support_matrix.qmd

@@ -7,8 +7,6 @@ hide:
 
 ```{python}
 #| echo: false
-from pathlib import Path
-
 import pandas as pd
 
 import ibis

ibis/backends/base/sqlglot/__init__.py

@@ -35,10 +35,12 @@ def dialect(self) -> sg.Dialect:
 
     @classmethod
     def has_operation(cls, operation: type[ops.Value]) -> bool:
-        # singledispatchmethod overrides `__get__` so we can't directly access
-        # the dispatcher
-        dispatcher = cls.compiler.visit_node.register.__self__.dispatcher
-        return dispatcher.dispatch(operation) is not dispatcher.dispatch(object)
+        compiler = cls.compiler
+        method = getattr(compiler, f"visit_{operation.__name__}", None)
+        return method is not None and method not in (
+            compiler.visit_Undefined,
+            compiler.visit_Unsupported,
+        )
 
     def _fetch_from_cursor(self, cursor, schema: sch.Schema) -> pd.DataFrame:
         import pandas as pd