Fast text.dat string references, more localization support via JS and @-codes

sbbs_t::mnemonics() now supports @-codes immediately following the tilde
(e.g. ~@Yes@) to use the first character of a dynamically-replaced
(e.g. localized/translated) text string as a command key.
Fixed issue in sbbs_t::mnemonics if multiple @-codes were specified in the
string, but no space characters (just noticed this bug while reviewing).

Created: sbbs_t::expand_atcodes() - used by sbbs_t::mnemonics(), exposed in
JS as bbs.expand_atcodes().

bbs.text and system.text now have properties: each text string ID is the
property name and the text index number (1-based) is the property value -
enabling fast text string lookup without load/require text.js

Constified sbbs_t::atcode() and attrstr() - not too painful

Deprecated YNQP text.dat string in favor of (the first character of)
  "Yes" and "No" text strings and new strings: "Quit", and "PasswordChar"

Other text.dat changes: ListKey->List, AllKey->Unused853
More new text.dat strings: Which, Next, Previous, Language, LANG
Deprecate the "AllKey" text string
Create/use sbbs_t::all_key() - first character of "All" text string

New JS console properties that present common (possibly localized) command
keys: yes_key, no_key, quit_key, all_key, list_key, next_key, prev_key