Fix encoding of file paths in chat links

[DanTup]

This fixes #2388, however the encoding format feels odd to me. I've opened microsoft/vscode#200965 to get some clarification about whether this is correct (and if so, hopefully it can be better documented).

fixed_links.mp4

@kalanchan

Test plan

• Open a chat
• Mention some files with @
• Ensure clicking the filenames in the chat message correctly open the files (note: the filenames in the collapsed set are handled differently, the issue was specifically with the converted links in the chat message)
• Also: verify the changes to tests for the new expected format are sensible and match what's discussed in command: links in markdown don't handle backslashes microsoft/vscode#200965 (at the time of writing there has been no response, so we may want to wait for one before merging this?)

[dominiccooney]

Nice work.