Add support for double underlines

This adds support for double underlines using the colon separated escape
sequence `CSI 4 : 2 m`.

Alacritty will now also always fallback to the normal underline in case
any of the other underlines like the undercurl are specified. The escape
sequence `CSI 4 : 0 m` can now be used to clear all underlines.

Some terminals support `CSI 21 m` for double underline, but since
Alacritty already uses that as cancel bold which is a little more
consistent, that behavior has not changed. So the colon separated
variant must be used.

alacritty_terminal/tests/ref.rs

@@ -66,6 +66,7 @@ ref_tests! {
     saved_cursor
     saved_cursor_alt
     sgr
+    underline
 }
 
 fn read_u8<P>(path: P) -> Vec<u8>

alacritty_terminal/tests/ref/underline/alacritty.recording

@@ -0,0 +1,13 @@
+[undeadleech@undeadlap underline]$ 
[undeadleech@undeadlap underline]$ echo -e "\e[4mUNDERLINED\e[0m"
+UNDERLINED
+[undeadleech@undeadlap underline]$ echo -e "\e[4:1mUNDERLINED\e[4:0m"
+[4:1mUNDERLINED[4:0m
+[undeadleech@undeadlap underline]$ echo -e "\e[4:2mUNDERLINED\e[24m"
+[4:2mUNDERLINED
+[undeadleech@undeadlap underline]$ echo -e "\e[4:3mUNDERLINED\e[21m";mUNDERLINED\e[21m"2mUNDERLINED\e[21m"1mUNDERLINED\e[21m"m"m"0m"
+[4:3;21mUNDERLINED
+[undeadleech@undeadlap underline]$ echo -e "\e[4;4:2mUNDERLINED\e[0m"
+[4;4:2mUNDERLINED
+[undeadleech@undeadlap underline]$ echo -e "\e[4;4:2mUNDERLINED\e[0m":;4:2mUNDERLINED\e[0m"1;4:2mUNDERLINED\e[0m";4:2mUNDERLINED\e[0m"2;4:2mUNDERLINED\e[0m"mUNDERLINED\e[0m"1mUNDERLINED\e[0m"
+[4:2;4:1mUNDERLINED
+[undeadleech@undeadlap underline]$ 

alacritty_terminal/tests/ref/underline/config.json

@@ -0,0 +1 @@
+{"history_size":0}

alacritty_terminal/tests/ref/underline/size.json

@@ -0,0 +1 @@
+{"width":939.0,"height":503.0,"cell_width":8.0,"cell_height":16.0,"padding_x":5.0,"padding_y":3.0,"dpr":1.0}