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>

ale_linters/nix/statix.vim

@@ -0,0 +1,18 @@
+scriptencoding utf-8
+" Author: David Houston <houstdav000>
+" Description: statix analysis and suggestions for Nix files
+
+call ale#Set('nix_statix_check_executable', 'statix')
+call ale#Set('nix_statix_check_options', '')
+
+function! ale_linters#nix#statix#GetCommand(buffer) abort
+    return '%e check -o errfmt --stdin'
+    \   . ale#Pad(ale#Var(a:buffer, 'nix_statix_check_options'))
+endfunction
+
+call ale#linter#Define('nix', {
+\   'name': 'statix',
+\   'executable': {b -> ale#Var(b, 'nix_statix_check_executable')},
+\   'command': function('ale_linters#nix#statix#GetCommand'),
+\   'callback': 'ale#handlers#statix#Handle',
+\})