webview resource path is incorrect in some cases

[BroKun]

Description

We fixed the #5503 problem before, which brought new problems.
On some plugins, such as vscode-python, their webview already contains a base tag.

<base href="vscode-resource:/pathToExtension/">
<script type="text/javascript" src="vscode-resource:/pathToExtensionr/index_bundle.js"></script>

If there is no slash prefix, you will get the wrong path.

webview/pathToExtension/webview/pathToExtension/index_bundle.js

Reproduction Steps

1. download vscode-python to the plugin directory.
2. open the interactive python window via a notebook file.

OS and Theia version:
ALL
Theia master branch

Diagnostics:
I can't find a way to support proxy mode in the case of only modifying the front end. I think we can add some configuration to support it.

[akosyakov]

@BroKun i believe it is a dup of #6282 (comment) and we should so something similar to #6282 (comment)