[VarDumper] Sfdump JS ns and VarCloner replacing Ext+PhpCloner

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12140 
| License       | MIT
| Doc PR        | -

As VarDumper is independant of the WDT, this patch creates a new JavaScript namespace used for HTML dumps.