feat(term size): add max-font-size mode 新增最大字型大小模式

[IepIweidieng]

The max-font-size mode behaviors like the fixed-font-size mode in that a basic font size can be specified by the user.
max-font-size（最大字型大小）模式很類似於 fixed-font-size（固定字型大小）模式，它們的基本字型大小都可由使用者指定。

However, when the window is too narrow or too short, fixed-font-size just lets the terminal screen overflow the window, while max-font-size shrinks the font to keep the screen inside the window.
但，當視窗過窄或過矮，fixed-font-size（固定字型大小）會直接讓終端機畫面超出視窗，而 max-font-size（最大字型大小）會縮小字體讓畫面維持在視窗內。

This feature should benefit users who use PttChrome on a relatively small screen (e.g., mobile/tablet users) and want to maximize the use of the limited space of their screen for reading without manually tweaking the terminal size in the fixed-term-size mode.
這個功能應有利於在相對小的螢幕（例如手機或平板電腦）上使用 PttChrome，且想儘量利用閱讀時有限的螢幕空間，又不想手動調整 fixed-term-size（固定終端機大小）中的終端機寬高設定的使用者。

Video for demonstration 展示影片:

DreamBBS-v3.0.2021-04-27.pttchrome.max-font-size.resizing.mp4

Note: After the recording of the video, the options for specifying the minimum height and width of the terminal screen size for max-font-size were dropped for the convenience of user. The minimum terminal screen size is hardcoded as 80 cols x 24 rows.
注意：在錄完該影片後，爲了方便使用者，移除了 max-font-size（最大字型大小）中用來指定終端機的最小高度與寬度的選項。終端機的最小寬高寫死成 80 縱列 × 24 橫行。