Create optional error code to flag missing return type

[denisekaur]

Fixes #15127

-This adds a new error code that flags when missing a return type on an annotation.
-This commit also contains documentation and tests for the error code.
-You can test this error code by using: mypy --disallow-incomplete-defs

[github-actions]

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

werkzeug (https://github.com/pallets/werkzeug)
- tests/test_serving.py:44: error: Function is missing a return type annotation  [no-untyped-def]
+ tests/test_serving.py:44: error: Function is missing a return type annotation  [no-incomplete-def]
- tests/test_serving.py:44: error: Function is missing a type annotation for one or more arguments  [no-untyped-def]
+ tests/test_serving.py:44: error: Function is missing a type annotation for one or more arguments  [no-incomplete-def]
- tests/test_http.py:378: error: Function is missing a type annotation for one or more arguments  [no-untyped-def]
+ tests/test_http.py:378: error: Function is missing a type annotation for one or more arguments  [no-incomplete-def]
- tests/test_formparser.py:367: error: Function is missing a return type annotation  [no-untyped-def]
+ tests/test_formparser.py:367: error: Function is missing a return type annotation  [no-incomplete-def]

rich (https://github.com/Textualize/rich)
+ rich/progress.py:241: error: Unused "type: ignore[no-untyped-def]" comment  [unused-ignore]
+ rich/progress.py:241: error: Function is missing a return type annotation  [no-incomplete-def]
+ rich/progress.py:241: note: Error code "no-incomplete-def" not covered by "type: ignore" comment

steam.py (https://github.com/Gobot1234/steam.py)
- steam/_const.py:64: error: Function is missing a return type annotation  [no-untyped-def]
+ steam/_const.py:64: error: Function is missing a return type annotation  [no-incomplete-def]
- steam/utils.py:149: error: Function is missing a type annotation for one or more arguments  [no-untyped-def]
+ steam/utils.py:149: error: Function is missing a type annotation for one or more arguments  [no-incomplete-def]
- steam/utils.py:153: error: Function is missing a type annotation for one or more arguments  [no-untyped-def]
+ steam/utils.py:153: error: Function is missing a type annotation for one or more arguments  [no-incomplete-def]
- steam/utils.py:156: error: Function is missing a type annotation for one or more arguments  [no-untyped-def]
+ steam/utils.py:156: error: Function is missing a type annotation for one or more arguments  [no-incomplete-def]
- steam/utils.py:194: error: Function is missing a type annotation for one or more arguments  [no-untyped-def]
+ steam/utils.py:194: error: Function is missing a type annotation for one or more arguments  [no-incomplete-def]
- steam/utils.py:198: error: Function is missing a type annotation for one or more arguments  [no-untyped-def]
+ steam/utils.py:198: error: Function is missing a type annotation for one or more arguments  [no-incomplete-def]
- steam/utils.py:201: error: Function is missing a type annotation for one or more arguments  [no-untyped-def]
+ steam/utils.py:201: error: Function is missing a type annotation for one or more arguments  [no-incomplete-def]

urllib3 (https://github.com/urllib3/urllib3)
+ src/urllib3/util/url.py:100: error: Unused "type: ignore" comment  [unused-ignore]
+ src/urllib3/util/url.py:100: error: Function is missing a return type annotation  [no-incomplete-def]
+ src/urllib3/util/url.py:100: note: Error code "no-incomplete-def" not covered by "type: ignore" comment
+ dummyserver/server.py:153: error: Unused "type: ignore" comment  [unused-ignore]
+ dummyserver/server.py:153: error: Function is missing a type annotation for one or more arguments  [no-incomplete-def]
+ dummyserver/server.py:153: note: Error code "no-incomplete-def" not covered by "type: ignore" comment

[ahzamm]

Hi @denisekaur, I would appreciate it if you could elaborate on the functionality or behavioral changes introduced by this pull request.