Fix live preview breaking on # in file or folder names

[Tech Guy (lukiod)]

Bug

PathUtil.EscapePathParts escapes each path segment with encodeURI, which deliberately leaves # (and a few other reserved characters like ?, &) unescaped — it's designed for encoding a full URI, where those characters are structural delimiters, not for encoding an individual path segment.

A file or folder name containing # therefore produces a preview URL where the browser treats everything from # onward as a fragment and never sends it to the server, so the file isn't found — the server falls back to showing the parent directory listing instead.

Fix

Switched EscapePathParts/UnescapePathParts to encodeURIComponent/decodeURIComponent, which escape (and correctly reverse) all reserved characters in a path segment rather than only some of them.

Verification

EscapePathParts/UnescapePathParts don't touch the VS Code API directly, so I could reproduce the exact old vs. new behavior standalone with Node's own encodeURI/encodeURIComponent:

--- old (buggy) ---
escapeOld('#folder/index.html') -> '#folder/index.html'   // # survives, browser truncates here
--- new (fixed) ---
escapeNew('#folder/index.html') -> '%23folder/index.html'
unescapeNew(that)                -> '#folder/index.html'  // round-trips correctly

Added 3 regression tests to src/test/suite/pathUtil.test.ts following the existing file's describe/it pattern, covering escape, round-trip, and a leading #. tsc -p ./ and eslint src are both clean. I wasn't able to run the full Electron-based extension-host test suite in this environment (no Xvfb, and installing one needs root here), so I'm disclosing that rather than claiming it — the new tests are pure (no vscode.* calls, matching the neighboring getEndpointParent tests in the same file) so they should run cleanly in CI.

Fixes #750