terminal: fix fgColor/bgColor commands [backport] (#15554)

Since #8296, fgSetColor is no longer a global. These commands were
probably left out from the change as an oversight, so some tests have
been added to make sure this won't happen again.

(cherry picked from commit d1af9587b8)

lib/pure/terminal.nim

@@ -687,10 +687,9 @@ template styledEchoProcessArg(f: File, color: Color) =
 template styledEchoProcessArg(f: File, cmd: TerminalCmd) =
   when cmd == resetStyle:
     resetAttributes(f)
-  when cmd == fgColor:
-    fgSetColor = true
-  when cmd == bgColor:
-    fgSetColor = false
+  elif cmd in {fgColor, bgColor}:
+    let term = getTerminal()
+    term.fgSetColor = cmd == fgColor
 
 macro styledWrite*(f: File, m: varargs[typed]): untyped =
   ## Similar to ``write``, but treating terminal style arguments specially.

tests/stdlib/tterminal.nim

@@ -0,0 +1,8 @@
+discard """
+  action: compile
+"""
+
+import terminal, colors
+
+styledEcho fgColor, colRed, "Test"
+styledEcho bgColor, colBlue, "Test"