Allow keyword errors for codec decoders

[jw9829]

Fixes #16441.

The decoder already accepts errors= at runtime. Move the / so input stays positional-only and errors can be passed by keyword.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

python-chess (https://github.com/niklasf/python-chess)
+ error: INTERNAL ERROR -- Please try using mypy master on GitHub:
+ https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
+ Please report a bug at https://github.com/python/mypy/issues
+ version: 2.3.0
+ note: use --pdb to drop into pdb
+ Traceback (most recent call last):
+   File "/__main__.py", line 16, in console_entry
+     main()
+     ~~~~^^
+   File "mypy/main.py", line 154, in main
+   File "mypy/main.py", line 244, in run_build
+     res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
+   File "mypy/build.py", line 385, in build
+   File "mypy/build.py", line 1908, in create_metastore
+     mds: MetadataStore = SqliteMetadataStore(
+   File "mypy/metastore.py", line 200, in __init__
+   File "mypy/metastore.py", line 174, in connect_db
+     db.execute("PRAGMA journal_mode=WAL")
+ sqlite3.OperationalError: database is locked
+ 

async-utils (https://github.com/mikeshardmind/async-utils)
- src/async_utils/_merge_gens.py:211: error: Argument 2 to "anext" has incompatible type "Task[T] | None"; expected "T"  [arg-type]
- src/async_utils/_merge_gens.py:262: error: Argument 2 to "anext" has incompatible type "Task[T] | None"; expected "T"  [arg-type]
- src/async_utils/_merge_gens.py:314: error: Argument 2 to "anext" has incompatible type "Task[T] | None"; expected "T"  [arg-type]
+ error: INTERNAL ERROR -- Please try using mypy master on GitHub:
+ https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
+ Please report a bug at https://github.com/python/mypy/issues
+ version: 2.3.0
+ note: use --pdb to drop into pdb
+ Traceback (most recent call last):
+   File "/__main__.py", line 16, in console_entry
+     main()
+     ~~~~^^
+   File "mypy/main.py", line 154, in main
+   File "mypy/main.py", line 244, in run_build
+     res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
+   File "mypy/build.py", line 385, in build
+   File "mypy/build.py", line 1908, in create_metastore
+     mds: MetadataStore = SqliteMetadataStore(
+   File "mypy/metastore.py", line 200, in __init__
+   File "mypy/metastore.py", line 174, in connect_db
+     db.execute("PRAGMA journal_mode=WAL")
+ sqlite3.OperationalError: database is locked
+