### Description

Addresses #5718 

The web build of Positron was showing the "No interpreters found" dialog
to users even if an interpreter was found. This bug is only present on
the web build because there are two extension hosts for the web build.
VS Code has a 'web extension host' that runs installed extensions in the
browser. Thus the Positron web build has this additional 'web extension
host'.

For our case, the 'web extension host' will never find any interpreters
during the runtime startup process. When the 'web extension host'
completes discovery before the extension host does, our list of
registered runtimes will be empty which causes the "No interpreters
found" dialog to be shown.

The runtime startup service needs to know about the existence of all
extension hosts and wait for all of the extension hosts to complete
their own language runtime discovery.

### Release Notes


#### New Features

- N/A

#### Bug Fixes

- The web build of Positron no longer shows the "No interpreters found"
dialog when interpreters are found. The runtime startup process now
waits for all extension hosts to complete runtime discovery.


### QA Notes
- [ ] Verify the "No interpreters found" dialog is not shown when there
are interpreters.
- [ ] Verify the "No interpreters found" dialog is shown only ONCE when
there aren't any interpreters.
  -  The following scenarios cover when the dialog can be shown:
    - There are no Python/R installations at all
- There are only outdated versions of Python/R installed which we do not
support, e.g. only having R 4.1 installed on the machine (we currently
support 4.2+)
- The Python/R installations are in hidden locations that Positron does
not check

### Screenshots

**Web Build - No Interpreters**


https://github.com/user-attachments/assets/5247d580-5eb7-44d5-8b75-1ff7b1f29565


**Web Build - Interpreters Found**


https://github.com/user-attachments/assets/adaa925b-5b0b-4f2e-bd7b-2690853138af