Implement statix Linter and Fixer (#3969)

* Add Statix for Linting

Add `statix check` as a linter. Provides a simple set of definition
tests additionally. Variable names specify "check" to allow for later
addition of `statix fix` as a formatter once stream support is added.

Signed-off-by: David Houston <houstdav000@gmail.com>

* Fixup Supported Tools List

I didn't realise there were two separate lists of tools, so add statix
to the other list. Also, remembered "S" comes after "R", and so
re-ordered it.

Signed-off-by: David Houston <houstdav000@gmail.com>

* Fix statix Test File

I refactored the variables for statix to allow for writing a fixer
later, and forgot to update them in the test, so update them now. Also
remove a stray "i", add missing space before checks

Signed-off-by: David Houston <houstdav000@gmail.com>

* Update Output Stream for v0.4.0

statix v0.4.0 provides a breaking change of output stream from stderr to
stdout.

Signed-off-by: David Houston <houstdav000@gmail.com>

* Add statix fix Fixer

Implement statix fix as a fixer for simple Nix antipatterns.

Signed-off-by: David Houston <houstdav000@gmail.com>

* Fix statix Fixer Tests

Fix the statix fixer tests by removing the unnecessary
'read_temporary_file' value from the command, since it simply uses the
default value.

Signed-off-by: David Houston <houstdav000@gmail.com>

* Add statix Handler Test

Add a test for the statix handler per @hsanson's request.

Signed-off-by: David Houston <houstdav000@gmail.com>

* Fix to run only on stdin for linting

Signed-off-by: David Houston <houstdav000@gmail.com>

test/handler/test_statix_handler.vader

@@ -0,0 +1,16 @@
+Execute(The statix handler should handle statix output):
+  call ale#test#SetFilename('flake.nix')
+
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 46,
+  \     'type': 'W',
+  \     'col': 13,
+  \     'code': '3',
+  \     'text': 'This assignment is better written with `inherit`'
+  \   },
+  \ ],
+  \ ale#handlers#statix#Handle(bufnr(''),
+  \   '<stdin>>46:13:W:3:This assignment is better written with `inherit`'
+  \)