Merge settings clangd.arguments from user and workspace

[zhang-stephen]

As the screenshot shows, I add different arguments for clangd in user default settings and workspace settings.

User default settings:

{
    "clangd.arguments": [
        "-j=6",
        "--all-scopes-completion",
        "--background-index",
        "--completion-style=bundled",
        "--enable-config",
        "--pch-storage=memory",
        "--offset-encoding=utf-16",
        "--log=info",
        "--header-insertion=iwyu",
    ]
}

Workspace settings:

{
    "clangd.arguments": [
        "--query-driver=/home/p8zhang/workspace/l1low/sysroot/thor/sysroots/x86_64-wrlinuxsdk-linux/usr/lib/aarch64-wrs-linux/g*"
    ]
}

I think this could be merged automatically, instead of a warning. I could keep the common arguments from different workspaces if so. I think it's more useful.

[i-ky]

If clangd.arguments was split into multiple options as suggested in #50, merging would happen "automatically" following usual VS Code setting precedence. There are not just user and workspace settings to merge, there are also remote settings and folder settings in multi-root workspaces. If merging is done by extension itself, it may be tricky to implement and behaviour may end up inconsistent with the rest of VS Code ecosystem.