[Task]: Set up vscode-extension-tester Correctly

# Task: Set up vscode-extension-tester Correctly

## Description

First step as outlined in https://github.com/ctc-oss/daffodil-vscode/discussions/129#discussioncomment-14500940.

To summarize all relevant discussion items so far. In https://github.com/apache/daffodil-vscode/issues/1290, an exploratory issue seeing if we're able to integrate vscode-extension tester, we encountered many issues. Issues that were ran into involved configuration (extest commands not working as expected), inconsistent test behavior, requiring sleeps, and that prior investigative efforts were done with inconsistencies noted. It is also noted in the ticket that prior attempts have been done and issues were encountered https://github.com/apache/daffodil-vscode/issues/1290#issuecomment-3207358512.

An overall summary of #1290 can be found https://github.com/ctc-oss/daffodil-vscode/discussions/129#discussioncomment-14500773

Some questions to consider: Are we creating a VS Code test instance where it doesn't bleed into the VS Code instance the developer is working off of? Should we use the CLI version of the extension tester or would using the Typescript API be better?

## Acceptance Criteria

- [ ] VS Code Extension Tester is installed
- [ ] Setting up a test instance of VS Code doesn't bleed into the user's VS Code instance
- This can be done either with the API or CLI version. API hasn't been tested yet.
- [ ] Behavior is consistent

## Additional Context

Refer to https://github.com/ctc-oss/daffodil-vscode/discussions/129#discussion-8110673,