fix(typing): Add missing float to IntoExpression alias

[dangotbanned]

Discovered during #3600

21d13e7

Will fix these false positives

cmd [3] | mypy altair tests
tests\vegalite\v5\test_api.py:602: error: Argument 2 to "if_" of "expr" has incompatible type "int"; expected "bool | str | OperatorMixin | dict[str, Any] | None"  [arg-type]
        expected = alt.expr.if_(alt.datum.b >= 0, 10, -20)
                                                  ^~
tests\vegalite\v5\test_api.py:602: error: Argument 3 to "if_" of "expr" has incompatible type "int"; expected "bool | str | OperatorMixin | dict[str, Any] | None"  [arg-type]
        expected = alt.expr.if_(alt.datum.b >= 0, 10, -20)
                                                      ^~~
tests\examples_arguments_syntax\one_dot_per_zipcode.py:14: error: Argument 2 to "substring" of "expr" has incompatible type "int"; expected "bool | str | OperatorMixin | dict[str, Any] | None"  [arg-type]
        "leading digit", alt.expr.substring(alt.datum.zip_code, 0, 1)
                                                                ^
tests\examples_arguments_syntax\one_dot_per_zipcode.py:14: error: Argument 3 to "substring" of "expr" has incompatible type "int"; expected "bool | str | OperatorMixin | dict[str, Any] | None"  [arg-type]
        "leading digit", alt.expr.substring(alt.datum.zip_code, 0, 1)
                                                                   ^
Found 4 errors in 2 files (checked 374 source files)