[DetSys#1182] treewide: add editorconfig and fix

This commit isn't cherry-picked from upstream. Instead, the
.editorconfig file was copied as-is from the upstream PR. Then,
`eclint -fix .` was ran to make everything conform to editorconfig. This
should be identical in impact, but it saves a lot of headache from
resolving useless merge conflicts.

Upstream-PR: https://github.com/DeterminateSystems/nix-installer/pull/1182
Change-Id: I22f03d18b3d685ff16b08bf8df0720e0f796d501

.editorconfig

@@ -0,0 +1,42 @@
+root = true
+
+# Unix-style newlines with a newline ending every file, utf-8 charset
+[*]
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+charset = utf-8
+
+# Rust
+[*.rs]
+indent_style = space
+
+# Misc
+[*.{yaml,yml,nix,json,sh,service,socket,toml,te}]
+insert_final_newline = true
+indent_style = space
+indent_size = 2
+
+[*.md]
+insert_final_newline = true
+indent_style = space
+
+[*.plist]
+indent_style = tab
+
+[*.ps1]
+indent_style = space
+indent_size = 4
+
+[Cargo.lock]
+indent_style = space
+indent_size = 1
+
+# selinux
+[*.pp]
+charset = unset
+end_of_line = unset
+indent_size = unset
+indent_style = unset
+insert_final_newline = unset
+trim_trailing_whitespace = unset