Problem:    Vim9: cannot assign to a global variable on the command line.
Solution:   Allow using :vim9cmd for assignment on the command line.

Problem:    Test fails because of new error message.
Solution:   Avoid the test fails.

Problem:    "pattern not found" for :global is not an error message.
Solution:   In Vim9 script make this an actual error, so that try/catch can be
            used as expected.

Problem:    Vim9: outdated "autocmd nested" still works.
Solution:   Do not accept the :autocmd argument "nested" without "++" in Vim9
            script.

Problem:    When there is a partially matching map and modifyOtherKeys is
            active a full map may not work.
Solution:   Only simplify modifiers when there is no matching mapping.
            (closes #8792)

Problem:    Vim9: there is no point in supporting :Print and :mode.
Solution:   Do not recognize :Print and :mode as commands. (closes #9870)

Problem:    In the GUI a modifier is not recognized for the key typed after
            CTRL-X, which may result in a mapping to be used. (Daniel
            Steinberg)
Solution:   Recognize a modifier starting with CSI. (closes #9889)

Problem:    With 'showbreak' set and after the end of the line the cursor
            may be displayed in the wrong position.
Solution:   Do not apply 'showbreak' after the end of the line. (closes #9884)

Problem:    Vim9: can declare a global variable on the command line.
Solution:   Disallow declaring a variable on the command line. (closes #9881)

Problem:    Vim9: at the script level declarations leak from try block to
            catch and finally block.
Solution:   End the block and start a new one. (closes #9883)