Add support for fish_trace variable to trace execution

This adds support for `fish_trace`, a new variable intended to serve the
same purpose as `set -x` as in bash. Setting this variable to anything
non-empty causes execution to be traced. In the future we may give more
specific meaning to the value of the variable.

The user's prompt is not traced unless you run it explicitly. Events are
also not traced because it is noisy; however autoloading is.

Fixes #3427

tests/checks/trace.fish

@@ -0,0 +1,72 @@
+# RUN: %fish %s
+
+echo untraced
+# CHECK: untraced
+
+set fish_trace 1
+
+for i in 1 2 3
+    echo $i
+end
+
+# CHECK: 1
+# CHECK: 2
+# CHECK: 3
+
+# CHECKERR: + for 1 2 3
+# CHECKERR: ++ echo 1
+# CHECKERR: ++ echo 2
+# CHECKERR: ++ echo 3
+# CHECKERR: + end for
+
+while true
+    and true
+    echo inside
+    break
+end
+
+# CHECK: inside
+
+# CHECKERR: + while
+# CHECKERR: + true
+# CHECKERR: + true
+# CHECKERR: ++ echo inside
+# CHECKERR: ++ break
+# CHECKERR: + end while
+
+while true && true
+    echo inside2
+    break
+end
+
+# CHECK: inside2
+
+# CHECKERR: + while
+# CHECKERR: + true
+# CHECKERR: + true
+# CHECKERR: ++ echo inside2
+# CHECKERR: ++ break
+# CHECKERR: + end while
+
+if true && false
+else if false || true
+    echo inside3
+else if will_not_execute
+end
+
+# CHECK: inside3
+
+# CHECKERR: + if
+# CHECKERR: + true
+# CHECKERR: + false
+# CHECKERR: + else if
+# CHECKERR: + false
+# CHECKERR: + true
+# CHECKERR: ++ echo inside3
+# CHECKERR: + end if
+
+set -e fish_trace
+# CHECKERR: + set -e fish_trace
+
+echo untraced
+# CHECK: untraced