Rework FindMatchingBrace() interface and implementation (#3319)

Instead of passing a single brace pair to FindMatchingBrace(), make it
traverse all brace pairs in buffer.BracePairs on its own.

This has the following advantages:

1. Makes FindMatchingBrace() easier to use, in particular much easier
   to use from Lua.

2. Lets FindMatchingBrace() ensure that we use just one matching brace -
   the higher-priority one. This fixes the following issues:

    ([foo]bar)
     ^

when the cursor is on `[`:

- Both `[]` and `()` pairs are highlighted, whereas the expected
  behavior is that only one pair is highlighted - the one that the
  JumpToMatchingBrace action would jump to.

- JumpToMatchingBrace action incorrectly jumps to `)` instead of
  `]` (which should take higher priority in this case).

In contrast, with `((foo)bar)` it works correctly.

internal/action/actions.go

@@ -1394,21 +1394,15 @@ func (h *BufPane) paste(clip string) {
 // JumpToMatchingBrace moves the cursor to the matching brace if it is
 // currently on a brace
 func (h *BufPane) JumpToMatchingBrace() bool {
-	for _, bp := range buffer.BracePairs {
-		r := h.Cursor.RuneUnder(h.Cursor.X)
-		rl := h.Cursor.RuneUnder(h.Cursor.X - 1)
-		if r == bp[0] || r == bp[1] || rl == bp[0] || rl == bp[1] {
-			matchingBrace, left, found := h.Buf.FindMatchingBrace(bp, h.Cursor.Loc)
-			if found {
-				if left {
-					h.Cursor.GotoLoc(matchingBrace)
-				} else {
-					h.Cursor.GotoLoc(matchingBrace.Move(1, h.Buf))
-				}
-				h.Relocate()
-				return true
-			}
+	matchingBrace, left, found := h.Buf.FindMatchingBrace(h.Cursor.Loc)
+	if found {
+		if left {
+			h.Cursor.GotoLoc(matchingBrace)
+		} else {
+			h.Cursor.GotoLoc(matchingBrace.Move(1, h.Buf))
 		}
+		h.Relocate()
+		return true
 	}
 	return false
 }