chore(demo): fix pattern to allow <1 decimals if can't be zero (#6903)

If an input in the demo can't be zero and a value <1 was given, the
check pattern would fail for it, and it'll show the warning that the
input isn't valid even though it is valid.

This slightly simplifies the regex creation as canBeZero adds `0+|` to
the pattern and the main pattern `([0-9]*[1-9][0-9]*)` is always used.
Then, if it can be a decimal, it uses the existing pattern but adds
`?(0(?=\.))?'` which makes the main pattern optional and instead adds
the optionally a `0` but only if it is followed by a `.`.

The main pattern `([0-9]*[1-9][0-9]*)` potentially could be simplified
into `[1-9]\d*` but it would make leading zeros invalid.

---------

Co-authored-by: Álvaro Velad Galván <ladvan91@hotmail.com>

demo/input.js

@@ -253,15 +253,21 @@ shakaDemo.NumberInput = class extends shakaDemo.TextInput {
     }
 
     this.input_.pattern = '(Infinity|';
+
     if (canBeZero) {
-      this.input_.pattern += '[0-9]*';
-    } else {
-      this.input_.pattern += '[0-9]*[1-9][0-9]*';
+      this.input_.pattern += '0+|';
     }
+
+    this.input_.pattern += '([0-9]*[1-9][0-9]*)';
+
     if (canBeDecimal) {
+      // strictly allow for 0.xxxx decimals
+      this.input_.pattern += '?(0(?=.))?';
       // TODO: Handle commas as decimal delimeters, for appropriate regions?
       this.input_.pattern += '(.[0-9]+)?';
     }
+
+
     this.input_.pattern += ')';
     if (canBeUnset) {
       this.input_.pattern += '?';