elixir: Fix mix linter and support warnings (#5140)

* elixir: Don't use temp dir for mix linter

The mix linter sets `MIX_BUILD_PATH` to a temporary directory on every
run, which is very slow and inefficient. We can just use the default
build path to speed this up.

The default directory is `_build` inside the project, this is also
automatically added to `.gitignore` by `mix new` so hopefully shouldn't
cause any unwanted clutter.

The `%s` argument is removed since `mix compile` doesn't actually
support passing a filename.

The output stream is changed to `stderr` since that's where the errors
appear.

Other options:
- `mix compile` has a `--no-compile` flag, but this misses most errors
  including basic syntax errors
- `mix format` has a `--dry-run` flag, but this only catches basic
  syntax errors and misses other errors like undefined functions

Fixes https://github.com/dense-analysis/ale/issues/3816

* elixir: Fix error parsing for mix linter and support warnings

The error parsing stopped working at some point, probably when Elixir
introduced its new error formatting in for 1.16 in 2023.

test/handler/test_mix_handler.vader

@@ -1,20 +1,85 @@
 Before:
   runtime ale_linters/elixir/mix.vim
+  call ale#test#SetFilename('/testplugin/lib/foo.ex')
 
 After:
   call ale#linter#Reset()
 
 Execute(The mix handler should parse lines correctly):
+  let input_lines = [
+  \ '    warning: variable "a" is unused (if the variable is not meant to be used, prefix it with an underscore)',
+  \ '    │',
+  \ ' 16 │     a = missing_var',
+  \ '    │     ~',
+  \ '    │',
+  \ '    └─ /testplugin/lib/foo.ex:16:5: Foo.hello/0',
+  \ '',
+  \ '    warning: variable "b" is unused (if the variable is not meant to be used, prefix it with an underscore)',
+  \ '    │',
+  \ ' 17 │     b = another_missing_var',
+  \ '    │     ~',
+  \ '    │',
+  \ '    └─ /testplugin/lib/foo.ex:17:5: Foo.hello/0',
+  \ '',
+  \ '    error: undefined variable "another_missing_var"',
+  \ '    │',
+  \ ' 17 │     b = another_missing_var',
+  \ '    │         ^^^^^^^^^^^^^^^^^^^',
+  \ '    │',
+  \ '    └─ /testplugin/lib/foo.ex:17:9: Foo.hello/0',
+  \ '',
+  \ '    error: undefined variable "missing_var"',
+  \ '    │',
+  \ ' 16 │     a = missing_var',
+  \ '    │         ^^^^^^^^^^^',
+  \ '    │',
+  \ '    └─ /testplugin/lib/foo.ex:16:9: Foo.hello/0',
+  \ '',
+  \ '    error: undefined variable "unrelated_var"',
+  \ '    │',
+  \ ' 16 │     a = unrelated_var',
+  \ '    │         ^^^^^^^^^^^',
+  \ '    │',
+  \ '    └─ /testplugin/lib/unrelated.ex:16:9: Foo.hello/0',
+  \ '',
+  \ '',
+  \ '== Compilation error in file /testplugin/lib/foo.ex ==',
+  \ '** (CompileError) /testplugin/lib/foo.ex: cannot compile module Foo (errors have been logged)',
+  \ ]
+
   AssertEqual
   \ [
   \   {
-  \     'bufnr': 347,
-  \     'lnum': 87,
-  \     'col': 0,
-  \     'text': 'undefined function update_in/4',
+  \     'bufnr': bufnr(''),
+  \     'lnum': 16,
+  \     'col': 5,
+  \     'type': 'W',
+  \     'text': 'variable "a" is unused (if the variable is not meant to be used, prefix it with an underscore)',
+  \     'detail': join(input_lines, "\n"),
+  \   },
+  \   {
+  \     'bufnr': bufnr(''),
+  \     'lnum': 17,
+  \     'col': 5,
+  \     'type': 'W',
+  \     'text': 'variable "b" is unused (if the variable is not meant to be used, prefix it with an underscore)',
+  \     'detail': join(input_lines, "\n"),
+  \   },
+  \   {
+  \     'bufnr': bufnr(''),
+  \     'lnum': 17,
+  \     'col': 9,
   \     'type': 'E',
+  \     'text': 'undefined variable "another_missing_var"',
+  \     'detail': join(input_lines, "\n"),
+  \   },
+  \   {
+  \     'bufnr': bufnr(''),
+  \     'lnum': 16,
+  \     'col': 9,
+  \     'type': 'E',
+  \     'text': 'undefined variable "missing_var"',
+  \     'detail': join(input_lines, "\n"),
   \   },
   \ ],
-  \ ale_linters#elixir#mix#Handle(347, [
-  \   '** (CompileError) apps/sim/lib/sim/server.ex:87: undefined function update_in/4'
-  \ ])
+  \ ale_linters#elixir#mix#Handle(bufnr(''), input_lines)