fix: use dunce::canonicalize to strip \\?\ prefix on Windows

[Jokerkern]

Problem

On Windows, std::fs::canonicalize returns paths prefixed with \\?\
(e.g. \\?\E:\project\...). When ELP matches a file opened in VS Code
against the app's source root, the comparison fails because VS Code URIs
resolve to plain paths like E:\project\... — the \\?\ prefix causes
a mismatch.

This causes every file to fall into the catch-all source root, which has
no app_data. The result is that all rebar3 projects on Windows are broken:

• L0003 "Unknown application" errors on every file
• Cross-file navigation (go-to-definition) not working
• Find references not working across files

Fix

Replace fs::canonicalize with dunce::canonicalize in into_abs_path.
The dunce crate is a well-known lightweight solution for this exact
problem, stripping the \\?\ prefix on Windows while behaving
identically to std::fs::canonicalize on all other platforms.

Testing

Verified on Windows 10 with a rebar3 project. All issues above are
resolved after this fix.

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