fix(language-service): do not treat file URIs as general URLs

[ayazhafiz]

In the past, the legacy (VE-based) language service would use a
UrlResolver instance to resolve file paths, primarily for compiler
resources like external templates. The problem with this is that the
UrlResolver is designed to resolve URLs in general, and so for a path
like /a/b/#c, #c is treated as hash/fragment rather than as part
of the path, which can lead to unexpected path resolution (f.x.,
resolve('a/b/#c/d.ts', './d.html') would produce 'a/b/d.html' rather
than the expected 'a/b/#c/d.html').

This commit resolves the issue by using Node's path module to resolve
file paths directly, which aligns more with how resources are resolved
in the Ivy compiler.

The testing story here is not great, and the API for validating a file
path could be a little bit prettier/robust. However, since the VE-based
language service is going into more of a "maintenance mode" now that
there is a clear path for the Ivy-based LS moving forward, I think it is
okay not to spend too much time here.

Closes angular/vscode-ng-language-service#892
Closes angular/vscode-ng-language-service#1001

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