Problem: Sourcing buffer lines is too complicated.
Solution: Simplify the code. Make it possible to source Vim9 script lines.
(Yegappan Lakshmanan, closes#9974)
Problem: Need to write script to a file to be able to source them.
Solution: Make ":source" use lines from the current buffer. (Yegappan
Lakshmanan et al., closes#9967)
Problem: Vim9: autoload mechanism doesn't fully work yet.
Solution: Define functions and variables with their autoload name, add the
prefix when calling a function, find the variable in the table of
script variables.
Problem: Vim9: crash when compiling heredoc lines start with comment.
Solution: Skip over NULL pointers. Do not remove comment and empty lines
when fetching function lines. (closes#6743)
Problem: Vim9: script cannot use line continuation like in a :def function.
Solution: Pass the getline function pointer to the eval() functions. Use it
for addition and multiplication operators.
Problem: Vim9: script reload test is disabled.
Solution: Compile a function in the context of the script where it was
defined. Set execution stack for compiled function. Add a test
that an error is reported for the right file/function.
Problem: Execution stack is incomplete and inefficient.
Solution: Introduce a proper execution stack and use it instead of
sourcing_name/sourcing_lnum. Create a string only when used.
