feat: Allow getting one chat at an index (#871)

# PR Checklist
- [ ] Have you checked if it works normally in all models? *Ignore this
if it doesn't use models.*
- [x] Have you checked if it works normally in all web, local, and node
hosted versions? If it doesn't, have you blocked it in those versions?
- [x] Have you added type definitions?

# Description

Getting one single chat at a specified index would help lighten Lua
scripts in certain scenarios such as "get the second last chat".

Currently I have to iterate (in `getFullChatMain`) the whole chat by
`getFullChat(id)` and then `fullChat[#fullChat - 1]`. With this PR, it
is simplified as `getChat(id, -2)`

Returns `nil` when no chat exists in the index.

src/ts/process/lua.ts

@@ -109,6 +109,20 @@ export async function runLua(code:string, arg:{
                 }
                 return alertSelect(value)
             })
+
+            luaEngine.global.set('getChatMain', (id:string, index:number) => {
+                const chat = luaEngineState.chat.message.at(index)
+                if(!chat){
+                    return JSON.stringify(null)
+                }
+                const data = {
+                    role: chat.role,
+                    data: chat.data,
+                    time: chat.time ?? 0
+                }
+                return JSON.stringify(data)
+            })
+
             luaEngine.global.set('setChat', (id:string, index:number, value:string) => {
                 if(!LuaSafeIds.has(id)){
                     return
@@ -153,12 +167,14 @@ export async function runLua(code:string, arg:{
                 let roleData:'user'|'char' = role === 'user' ? 'user' : 'char'
                 luaEngineState.chat.message.splice(index, 0, {role: roleData, data: value})
             })
+
             luaEngine.global.set('getTokens', async (id:string, value:string) => {
                 if(!LuaSafeIds.has(id)){
                     return
                 }
                 return await tokenize(value)
             })
+
             luaEngine.global.set('getChatLength', (id:string) => {
                 return luaEngineState.chat.message.length
             })
@@ -771,6 +787,10 @@ function luaCodeWarper(code:string){
     return `
 json = require 'json'
 
+function getChat(id, index)
+    return json.decode(getChatMain(id, index))
+end
+
 function getFullChat(id)
     return json.decode(getFullChatMain(id))
 end