Prefer const vs let for variables that are only declared once

Requires that variable declarations use `const` instead of `let` and `var` if possible.

This is mostly for code readability, performance wins are debatable.

src/CharWidth.ts

@@ -161,7 +161,7 @@ export const wcwidth = (function(opts: {nul: number, control: number}): (ucs: nu
       if (num < 127) {
         return 1;
       }
-      let t = table || init_table();
+      const t = table || init_table();
       if (num < 65536) {
         return t[num >> 4] >> ((num & 15) << 1) & 3;
       }