plugin/history-eternal: Use readonly instead of export

...and hide errors relating to setting already-readonly variables.

`plugin/history-eternal` does not need to force loading after `plugin/history` because both plugins will play nicely with read-only variables, and since we're overwritting and marking read-only then the intended result survives no matter which loads first.

plugins/available/history-eternal.plugin.bash

@@ -1,20 +1,17 @@
 # shellcheck shell=bash
 about-plugin 'eternal bash history'
 
-# Load after the history plugin
-# BASH_IT_LOAD_PRIORITY: 375
-
 # Modify history sizes before changing location to avoid unintentionally
 # truncating the history file early.
 
 # "Numeric values less than zero result in every command being saved on the history list (there is no limit)"
-export HISTSIZE=-1
+readonly HISTSIZE=-1 2> /dev/null
 
 # "Non-numeric values and numeric values less than zero inhibit truncation"
-export HISTFILESIZE='unlimited'
+readonly HISTFILESIZE='unlimited' 2> /dev/null
 
 # Use a custom history file location so history is not truncated
 # if the environment ever loses this "eternal" configuration.
 HISTDIR="${XDG_STATE_HOME:-${HOME?}/.local/state}/bash"
 [[ -d ${HISTDIR?} ]] || mkdir -p "${HISTDIR?}"
-export HISTFILE="${HISTDIR?}/history"
+readonly HISTFILE="${HISTDIR?}/history" 2> /dev/null