VSCode: Do not parse Windows file paths as URIs

[dominiccooney]

Fixes #2184. Uri.parse parses URIs, Uri.file takes a file path and turns it into a URI. For UNIX-style paths, Uri.parse works fine, but for Windows file paths, parsing them as URIs is wrong; the the drive separator is URL-encoded and the URI is unusable, etc.

Extension.Development.Host.CODE_OF_CONDUCT.md.-.lexical.-.Visual.Studio.Code.2023-12-08.11-59-54.mp4

Test plan

1. Open a workspace, start a chat
2. @-mention a file in the workspace
3. The file should be included in the chat context, Cody can see the content, etc.

[dominiccooney]

Reviewed our other uses of Uri.parse and they appear correct. They may handle files, but they are file URIs.

[abeatrix]

Oops thanks for catching this