Refresh dependencies, drop node-sass

`npm audit` revealed some vulnerabilities:

```

scss-tokenizer  <=0.4.2
Severity: high
Regular expression denial of service in scss-tokenizer - https://github.com/advisories/GHSA-7mwh-4pqv-wmr8
fix available via `npm audit fix --force`
Will install node-sass@4.5.3, which is a breaking change
node_modules/scss-tokenizer
  sass-graph  >=2.2.0
  Depends on vulnerable versions of scss-tokenizer
  node_modules/sass-graph
    node-sass  >=4.6.0
    Depends on vulnerable versions of sass-graph
    node_modules/node-sass

3 high severity vulnerabilities

```

We don't need node-sass, and sass-loader is just as happy using
another sass implementation. The preferred one is dart-sass which
is simply called sass in npm.

In the process, I also did an `npm update` just for good measure.

bindata/static/css/xterm.css

@@ -36,6 +36,7 @@
  */
 
 .xterm {
+    cursor: text;
     position: relative;
     user-select: none;
     -ms-user-select: none;
@@ -124,10 +125,6 @@
     line-height: normal;
 }
 
-.xterm {
-    cursor: text;
-}
-
 .xterm.enable-mouse-events {
     /* When mouse events are enabled (eg. tmux), revert to the standard pointer cursor */
     cursor: default;
@@ -178,3 +175,16 @@
 	z-index: 6;
 	position: absolute;
 }
+
+.xterm-decoration-overview-ruler {
+    z-index: 7;
+    position: absolute;
+    top: 0;
+    right: 0;
+    pointer-events: none;
+}
+
+.xterm-decoration-top {
+    z-index: 2;
+    position: relative;
+}