state man. API && modularisation

- adds functions
  - util_create_state
  - util_read_state
  - util_write_state
  - util_delete_state
for state management API. see docs/about_states.md for details.

- moves functions from _reve.sh to _reve_utils.sh to prevent circular
imports.

_reve_utils.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+
+# reve                              desktop environment framework
+# Yigid BALABAN <fyb@fybx.dev>                               2024
+
+# reve internal: _reve_utils
+# defines utility functions
+
+# VERY CRITICAL: change this if install.sh is updated
+reve_installation="$HOME/.local/bin/reve"
+
+util_readf() {
+  local filename=$1
+
+  if [[ -f "$filename" ]]; then
+    cat "$filename"
+  else
+    error E "util_readf" "File not found: $filename"
+    return 1
+  fi
+}
+
+error() {
+  local level=$1 location=$2 cause=$3
+  message="[reve] [$level] [$location] $cause"
+  echo "$message"
+  now=$(date -Iminutes)
+  echo "${now::-6} $message" >>"$reve_installation/reve.log"
+}