runtime(vim): Split Vim legacy and Vim9 script indent tests

runtime(vim): Split Vim legacy and Vim9 script indent tests

The indent test file is not highlighted correctly as it's currently a
syntactically erroneous mix of legacy and Vim9 script.  This is causing
spurious indent test failures as the indent script relies on syntax ID
tests to determine the context of lines being processed.

Splitting the tests into legacy and Vim9 specific files allows for
correct syntax highlighting and indenting.

closes: #16431

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>