Doesn't recognice def as functions

[Gurran]

Describe the bug

Does not recognize vim9 functions as functions and warns that return isn't inside a function block.
NOTE: this is inside a file with vim9script as the first line.

To Reproduce

vim9script

def MyFunction(): bool
    return true
enddef

Expected behavior
No error to be shown

Screenshots

[iamcco]

vimscript9 is not support yet. #78