Implemented 'set allow-duplicate-variables'

[Mijago]

I ran into the same issue as #1827 and decided to make a quick draft for it.
It's my first rodeo with rust (and I am not a big fan so far, but it may grow onto me), so I appreciate feedback.

This issue allows us to overwrite assignments, even when they are imported from other justfiles. Previously it would have thrown an error. We now can use set allow-duplicate-variables to enable this behavior (in the same style as the existing set allow-duplicate-recipes).

Example: Default behavior

a := "foo"
a := "bar" 
#  Example: Raises an exception

Example: Using the flag to overwrite a variable

set allow-duplicate-variables

a := "foo"
a := "bar"
# a is now "bar"

Example: Using the flag in an import

# imp.just
a := "foo"

# some.just
import "./imp.just"
set allow-duplicate-variables

a := "bar"
# a is now "bar"

Feedback appreciated

I am not quite happy with the detour I had to take with DepthAwareBinding. I introduced it because the assignments need to be aware of their depth, otherwise imported modules would overwrite assignments that appear later in the justfile. As I did not use rust before, I do not know the best practices for this in this language and just wanted to get the draft done (for now).

Some other thing

I ran into just just (hehe) a few months ago - and fell in love. Thanks for this tool!

[Mijago]

@casey Hi! I do not want to come off rude or impatient - but is there anything I should change in order to progress this PR further?

[casey]

Sorry for letting this sit! This looks great. I made a couple changes:

• I removed DepthAwareBinding and just added a depth, which is set to 0 when not needed, to Binding.
• I removed the modules_use_module_settings_variables test. Since there is no code specific to different settings which implements this, I don't think a new test is needed for each setting.