Add support for underline styles

[joelim-work]

• Fixes Is undercurl supportted? #1232

---

Add support for the following terminal escape sequences which set underline styles:

Escape sequence	Description
\033[4:0m	No underline
\033[4:1m	Single underline (same as \033[4m)
\033[4:2m	Double underline
\033[4:3m	Curly underline
\033[4:4m	Dotted underline
\033[4:5m	Dashed underline

Sample config for testing:

cmd test &{{
    lf -remote "send $id set cursorpreviewfmt \"\033[4:0m\""
    sleep 1
    lf -remote "send $id set cursorpreviewfmt \"\033[4:1m\""
    sleep 1
    lf -remote "send $id set cursorpreviewfmt \"\033[4:2m\""
    sleep 1
    lf -remote "send $id set cursorpreviewfmt \"\033[4:3m\""
    sleep 1
    lf -remote "send $id set cursorpreviewfmt \"\033[4:4m\""
    sleep 1
    lf -remote "send $id set cursorpreviewfmt \"\033[4:5m\""
    sleep 1
    lf -remote "send $id set cursorpreviewfmt \"\033[4m\""
}}