Reorganize command-line shims for Neovim

The previous shim has been renamed to `lua` because it is effectively
just Neovim acting as a Lua interpreter.  A new shim named `nvim` has
been added which acts as an interactive Neovim.

.busted

@@ -26,7 +26,7 @@
 return {
 	_all = {
 		coverage = false,
-		lua = './test/nvim-shim',
+		lua = './test/lua',
 		['exclude-tags'] = {'skip'},
 	},
 	default = {

test/nvim

@@ -0,0 +1,38 @@
+#!/bin/sh
+# SPDX-License-Identifier: Unlicense
+
+# This is free and unencumbered software released into the public domain.
+#
+# Anyone is free to copy, modify, publish, use, compile, sell, or distribute
+# this software, either in source code form or as a compiled binary, for any
+# purpose, commercial or non-commercial, and by any means.
+#
+# In jurisdictions that recognize copyright laws, the author or authors of
+# this software dedicate any and all copyright interest in the software to
+# the public domain.  We make this dedication for the benefit of the public
+# at large and to the detriment of our heirs and successors.  We intend this
+# dedication to be an overt act of relinquishment in perpetuity of all
+# present and future rights to this software under copyright law.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+# THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+# AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+#
+# For more information, please refer to <https://unlicense.org/>
+
+
+# A wrapper around Neovim which makes the editor launch in the isolated
+# environment.  This wrapper is meant for interactive use so the programmer can
+# poke around in a live Neovim process which is not affected by the
+# programmer's own configuration.
+
+# Set custom XDG base directory paths to isolate the test Neovim from the
+# user's own configuration and data files.
+export XDG_CONFIG_HOME='test/xdg/config/'
+export XDG_STATE_HOME='test/xdg/local/state/'
+export XDG_DATA_HOME='test/xdg/local/share/'
+
+nvim $@