Update Elixir.gitignore to match modern standards

Update Elixir.gitignore to match the upstream template for new Elixir projects.

a9021b1c8d/lib/mix/lib/mix/tasks/new.ex (L307-L331)

Remove `.fetch` which was also removed upstream (https://github.com/elixir-lang/elixir/pull/13871).

Preserve additional patterns added in https://github.com/github/gitignore/pull/2132, https://github.com/github/gitignore/pull/2611, and https://github.com/github/gitignore/pull/2872, adding comments to match the style of the rest of the file.

Elixir.gitignore

@@ -1,10 +1,32 @@
-/_build
-/cover
-/deps
-/doc
-/.fetch
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Temporary files, for example, from tests.
+/tmp/
+
+# If the VM crashes, it generates a dump, let's ignore it too.
 erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
 *.ez
+
+# Ignore package tarball (built via "mix hex.build").
+*.tar
+
+# Compiled files (usually in /_build/).
 *.beam
+
+# Sensitive configuration, common convention in older Phoenix applications.
 /config/*.secret.exs
+
+# Language Server temporary files.
 .elixir_ls/