Test for html src completion

[octref]

• macOS: @gushuro
• linux: @rebornix
• windows: @dbaeumer

Complexity: 2

Testing #2037

In this iteration we added path completions to html attribute values where applicable. For example <a href="">, <img src="">. The type completion is triggered each time you type /.

Known Limitation

• Path Completion does not work in HTML 5 unquoted attribute yet

Verify

• . and .. completion works as expected
• / shows file/folders from workspace root (test on multi root too)
• Symbolic link works
• For http or https or // links path completion is not triggered