Add base configuration

- Follow XDG standard: ~/.config and ~/.local folders
- Set environment variables and define aliases within ~/.config/shell
- Add installation script for easy setup and maintenance
  (i.e., `dotfiles-install` and `dotfiles-update`)
- Add README.md with info on the installation and general notes

.config/shell/env

@@ -0,0 +1,52 @@
+#!/bin/sh
+
+# Environment variables for all kinds of shells
+
+
+# Standard XDG base directories
+# See: https://wiki.archlinux.org/title/XDG_Base_Directory
+
+export XDG_CACHE_HOME="$HOME/.cache"
+export XDG_CONFIG_HOME="$HOME/.config"
+export XDG_DATA_HOME="$HOME/.local/share"
+export XDG_STATE_HOME="$HOME/.local/state"
+
+# Make up a XDG directory for binaries (that does not exist in the standard)
+export XDG_BIN_HOME="$HOME/.local/bin"
+
+
+# Convenient names for various places in the system,
+# shown here not only to be used but also document things
+
+export DOTFILES_DIR="$XDG_DATA_HOME/dotfiles"
+export DOTFILES_HTTPS="https://code.webartifex.de/alexander/dotfiles"
+export DOTFILES_SSH="git@git.webartifex.de:alexander/dotfiles.git"
+
+
+# Generic shell configs
+
+
+# Fall back to a widely known terminal type
+# if this host lacks the terminfo entry for `$TERM`
+# (e.g., `alacritty` over `ssh` on a fresh server)
+if _command_exists infocmp && ! infocmp "$TERM" >/dev/null 2>&1; then
+    export TERM=xterm-256color
+fi
+
+
+[ -t 0 ] && export GPG_TTY=$(tty)
+
+
+export EDITOR=nano
+
+
+export PAGER=less
+export LESS="--chop-long-lines --ignore-case --LONG-PROMPT --no-init --quit-if-one-screen --RAW-CONTROL-CHARS --status-column"
+
+
+[ -f "$XDG_CONFIG_HOME/shell/locale" ] && . "$XDG_CONFIG_HOME/shell/locale"
+
+
+# Move common tools' config and cache files into XDG directories
+
+export LESSHISTFILE="$XDG_STATE_HOME/less/history"