command/format: Use box drawing chars for rules in diagnostics

We were previously using some ASCII art to create some visual divisions
between parts of the diagnostic output. Now that we are requiring a UTF-8
terminal we can print out box drawing characters instead.

command/format/diagnostic.go

@@ -172,10 +172,10 @@ func Diagnostic(diag tfdiags.Diagnostic, sources map[string][]byte, color *color
 			sort.Strings(stmts) // FIXME: Should maybe use a traversal-aware sort that can sort numeric indexes properly?
 
 			if len(stmts) > 0 {
-				fmt.Fprint(&buf, color.Color("    [dark_gray]|----------------[reset]\n"))
+				fmt.Fprint(&buf, color.Color("    [dark_gray]├────────────────[reset]\n"))
 			}
 			for _, stmt := range stmts {
-				fmt.Fprintf(&buf, color.Color("    [dark_gray]|[reset] %s\n"), stmt)
+				fmt.Fprintf(&buf, color.Color("    [dark_gray]│[reset] %s\n"), stmt)
 			}
 		}
 

command/format/diagnostic_test.go

@@ -94,8 +94,8 @@ Whatever shall we do?
 
   on test.tf line 1:
    1: test [underline]source[reset] code
-    [dark_gray]|----------------[reset]
-    [dark_gray]|[reset] [bold]boop.beep[reset] is "blah"
+    [dark_gray]├────────────────[reset]
+    [dark_gray]│[reset] [bold]boop.beep[reset] is "blah"
 
 Whatever shall we do?
 `,
@@ -127,8 +127,8 @@ Whatever shall we do?
 
   on test.tf line 1:
    1: test [underline]source[reset] code
-    [dark_gray]|----------------[reset]
-    [dark_gray]|[reset] [bold]boop.beep[reset] has a sensitive value
+    [dark_gray]├────────────────[reset]
+    [dark_gray]│[reset] [bold]boop.beep[reset] has a sensitive value
 
 Whatever shall we do?
 `,
@@ -160,8 +160,8 @@ Whatever shall we do?
 
   on test.tf line 1:
    1: test [underline]source[reset] code
-    [dark_gray]|----------------[reset]
-    [dark_gray]|[reset] [bold]boop.beep[reset] is a string, known only after apply
+    [dark_gray]├────────────────[reset]
+    [dark_gray]│[reset] [bold]boop.beep[reset] is a string, known only after apply
 
 Whatever shall we do?
 `,
@@ -193,8 +193,8 @@ Whatever shall we do?
 
   on test.tf line 1:
    1: test [underline]source[reset] code
-    [dark_gray]|----------------[reset]
-    [dark_gray]|[reset] [bold]boop.beep[reset] will be known only after apply
+    [dark_gray]├────────────────[reset]
+    [dark_gray]│[reset] [bold]boop.beep[reset] will be known only after apply
 
 Whatever shall we do?
 `,