富文本消息显示大重构！！！

* 将所有的 custom element 以正确的方式重新编写
* 可以正确解析 Markdown 文本, 图片, 斜体文本元素且不会杂糅了
* 通过 DOM 操作使得所有的文本聚合在一起, 并且取消了消息自带的填充边距, 删除了原本消息内无法正常工作的 "无边框显示模式"
* 添加新的 custom-element: chat-text 和 chat-text-container

client/ui/custom-elements/chat-text-container.ts

@@ -0,0 +1,16 @@
+customElements.define('chat-text-container', class extends HTMLElement {
+    declare container: HTMLDivElement
+    constructor() {
+        super()
+
+        this.attachShadow({ mode: 'open' })
+    }
+    connectedCallback() {
+        const shadow = this.shadowRoot as ShadowRoot
+        
+        this.container = document.createElement('div')
+        this.container.style.padding = '13px'
+        shadow.appendChild(this.container)
+        this.container.innerHTML = this.innerHTML
+    }
+})

client/ui/custom-elements/chat-text.ts

@@ -0,0 +1,29 @@
+import { $ } from 'mdui'
+
+customElements.define('chat-text', class extends HTMLElement {
+    declare span: HTMLSpanElement
+    static observedAttributes = ['underline', 'em']
+    constructor() {
+        super()
+
+        this.attachShadow({ mode: 'open' })
+    }
+    connectedCallback() {
+        const shadow = this.shadowRoot as ShadowRoot
+        
+        this.span = document.createElement('span')
+        this.span.style.whiteSpace = 'pre-wrap'
+        this.span.style.fontSynthesis = 'style weight'
+        shadow.appendChild(this.span)
+
+        this.update()
+    }
+    attributeChangedCallback(_name: string, _oldValue: unknown, _newValue: unknown) {
+        this.update()
+    }
+    update() {
+        this.span.textContent = this.textContent
+        this.span.style.textDecoration = $(this).attr('underline') ? 'underline' : ''
+        this.span.style.fontStyle = $(this).attr('em') ? 'italic' : ''
+    }
+})