Fix R help url resolution on Server Web and Workbench

[sharon-wang]

Description

• addresses part of URL reshaping and proxying for Positron localhost URLs on Posit Workbench #4274
• fixes R help docs not loading in Positron Server and Positron on Workbench

Implementation Notes

• this adds an additional content rewriting step to the positronProxy extension, to replace any root-relative urls in the help HTML (urls that start with /) with the proxied path, so that those resources are loaded from the correct location
• the rewriting step unfortunately uses a simple regex which does not account for the full range of ways to specify a url in HTML -- it simply replaces the slash \ in instances of src="\ and href="\ with the proxyPath
• this rewriting seems sufficient for what R help outputs in some limited testing, and we don't currently have a way to properly parse and modify the HTML aside from string replacement, so we'll go with this for now and keep an eye on it for improvements/fixes

QA Notes

In Positron Server Web and Positron on Workbench:

1. Select an R interpreter
2. In the Console, get help for something, e.g. ?iris and hit Enter
3. What to expect in the Help Pane:
   • the pane should display the help content with styling that matches the current IDE theme
   • links to other help pages should load and work when clicked
   • there shouldn't be 404 Not Found errors in the Developer Console or Network tab for paths referenced in the help HTML

The Help Pane in Positron Desktop should continue to work.

Note that Python help issues on Server Web and Workbench are not addressed in this PR.

[softwarenerd]

LG!