Fix local filepath on POSIX

[Warchamp7]

Description

Fixes the leading / being trimmed on posix filesystems.

Motivation and Context

Want local files to work.

How Has This Been Tested?

Tested by Penwywern on Discord.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[Penwy]

For precision : test was done on ubuntu 24.04

[PatTheMav]

Before we merge a fix for just a single issue, it would be great if we could get confirmation for a full test suite of possible URLs being confirmed to work as expected, otherwise we just pile little fixes on top of each other (and I repeat my suggestion made offline to use std::filesystem:path, which has platform-specific implementations already).

[PatTheMav]

Doesn't break valid POSIX file paths anymore, but I haven't run a full gamut of checks for paths that are considered invalid by this.

[RytoEX]

Per off-thread discussion, going to merge this, but would like to see more comprehensive testing (a list of URLs tested and expected and actual outcomes).

[RytoEX]

On Windows 11, I tested the following supported scenarios (all should load and therefore PASS).

Local File:

PASS: C:/test/path/test.html
PASS: C:/test/path/spaces in path/test.html

URL:

PASS: file:///C:\test\path\test.html
PASS: file://C:\test\path\test.html
PASS: file:///C:\test\path\spaces in path\test.html
PASS: file://C:\test\path\spaces in path\test.html