fix(linux): convert StartupWMClass template values to lowercase

[TumnusVirus]

Linux .deb installer creates duplicate desktop shortcuts leading to startup crash loops and orphaned crashpad handlers on Ubuntu MATE
#327088

When VS Code or VS Code Insiders compiles on Linux, it sets StartupWMClass using standard camel-case names, but the underlying runtime engine launches as low-case code or code-insiders, breaking taskbar icon-grouping window mappings on Linux panels like Ubuntu MATE.

Whenever a new version of code or code-insiders is downloaded for me, it opens up the URL handler desktop instead of the main desktop. With this change, it should no longer happen. I've gone into the code.desktop / code-insiders.desktop folder and updated it to code or code-insiders and it no longer exhibits that behaviour.

[Copilot]

Pull request overview

Updates Linux packaging to align StartupWMClass with lowercase runtime identifiers.

Changes:

• Transforms the desktop template value for DEB, RPM, and Snap packages.
• Current transformation incorrectly produces triple hyphens for names such as Code - OSS.

Comments suppressed due to low confidence (2)

build/gulpfile.vscode.linux.ts:162

• The RPM path has the same invalid derivation: Code - OSS becomes code---oss, which cannot match the code-oss runtime WM class. Use the existing product application name rather than converting the display name.

			.pipe(replace('@@NAME_SHORT@@', product.nameShort.toLowerCase().replace(/ /g, '-')))

build/gulpfile.vscode.linux.ts:249

• The Snap desktop file will also get code---oss for the default product (and an analogous triple-hyphen Insiders value), so its StartupWMClass remains mismatched. Populate it from applicationName, which is already the runtime/package identifier.

			.pipe(replace('@@NAME_SHORT@@', product.nameShort.toLowerCase().replace(/ /g, '-')))