Add editor, prettier, and ignore dotfiles

.gitignore extended for TypeScript/Node build artifacts on top of the
prompts-repo template. .editorconfig and .prettierrc match the prompts
template (4-space indents, prose-wrap always for markdown).

.editorconfig

@@ -0,0 +1,12 @@
+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[Makefile]
+indent_style = tab