chore: improve markdown to html perf (#26644)

### What does this PR do?

I was looking at the [recent
support](https://github.com/oven-sh/bun/pull/26440) for markdown and did
some benchmarking against
[bindings](https://github.com/just-js/lo/blob/main/lib/md4c/api.js) i
created for my `lo` runtime to `md4c`. In some cases, Bun is quite a bit
slower, so i did a bit of digging and came up with this change. It uses
`indexOfAny` which should utilise `SIMD` where it's available to scan
ahead in the payload for characters that need escaping.

In
[benchmarks](https://gist.github.com/billywhizz/397f7929a8920c826c072139b695bb68#file-results-md)
I have done this results in anywhere from `3%` to `~15%` improvement in
throughput. The bigger the payload and the more space between entities
the bigger the gain afaict, which would make sense.

### How did you verify your code works?

It passes `test/js/bun/md/*.test.ts` running locally. Only tested on
macos. Can test on linux but I assume that will happen in CI anyway?

## main


![bun-main](https://github.com/user-attachments/assets/8b173b34-1f20-4e52-bb67-bb8b7e5658f3)

## patched


![bun-patch](https://github.com/user-attachments/assets/26bb600c-234c-4903-8f70-32f167481156)

src/md/html_renderer.zig

@@ -486,22 +486,27 @@ pub const HtmlRenderer = struct {
     }
 
     pub fn writeHtmlEscaped(self: *HtmlRenderer, txt: []const u8) void {
-        var start: usize = 0;
-        for (txt, 0..) |c, i| {
-            const replacement: ?[]const u8 = switch (c) {
-                '&' => "&",
-                '<' => "&lt;",
-                '>' => "&gt;",
-                '"' => "&quot;",
-                else => null,
+        var i: usize = 0;
+        const needle = "&<>\"";
+
+        while (true) {
+            const next = std.mem.indexOfAny(u8, txt[i..], needle) orelse {
+                self.write(txt[i..]);
+                return;
             };
-            if (replacement) |r| {
-                if (i > start) self.write(txt[start..i]);
-                self.write(r);
-                start = i + 1;
+            const pos = i + next;
+            if (pos > i)
+                self.write(txt[i..pos]);
+            const c = txt[pos];
+            switch (c) {
+                '&' => self.write("&amp;"),
+                '<' => self.write("&lt;"),
+                '>' => self.write("&gt;"),
+                '"' => self.write("&quot;"),
+                else => unreachable,
             }
+            i = pos + 1;
         }
-        if (start < txt.len) self.write(txt[start..]);
     }
 
     fn writeUrlEscaped(self: *HtmlRenderer, txt: []const u8) void {