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/display/bufwindow.go

@@ -392,28 +392,20 @@ func (w *BufWindow) displayBuffer() {
 	var matchingBraces []buffer.Loc
 	// bracePairs is defined in buffer.go
 	if b.Settings["matchbrace"].(bool) {
-		for _, bp := range buffer.BracePairs {
-			for _, c := range b.GetCursors() {
-				if c.HasSelection() {
-					continue
-				}
-				curX := c.X
-				curLoc := c.Loc
+		for _, c := range b.GetCursors() {
+			if c.HasSelection() {
+				continue
+			}
 
-				r := c.RuneUnder(curX)
-				rl := c.RuneUnder(curX - 1)
-				if r == bp[0] || r == bp[1] || rl == bp[0] || rl == bp[1] {
-					mb, left, found := b.FindMatchingBrace(bp, curLoc)
-					if found {
-						matchingBraces = append(matchingBraces, mb)
-						if !left {
-							if b.Settings["matchbracestyle"].(string) != "highlight" {
-								matchingBraces = append(matchingBraces, curLoc)
-							}
-						} else {
-							matchingBraces = append(matchingBraces, curLoc.Move(-1, b))
-						}
+			mb, left, found := b.FindMatchingBrace(c.Loc)
+			if found {
+				matchingBraces = append(matchingBraces, mb)
+				if !left {
+					if b.Settings["matchbracestyle"].(string) != "highlight" {
+						matchingBraces = append(matchingBraces, c.Loc)
 					}
+				} else {
+					matchingBraces = append(matchingBraces, c.Loc.Move(-1, b))
 				}
 			}
 		}