Placeholders should be expanded in `swift.buildPath`

[jakepetroules]

I'd like to be able to set that to something ending in ${workspaceFolderBaseName}, otherwise it isn't really possible to make it a user-level setting.

For example, something like ${homePath/spm-builds/${workspaceFolderBaseName}

[award999]

We should go through https://code.visualstudio.com/docs/reference/variables-reference and figure out which ones make sense for vscode-swift. Unfortunately VSCode does not make this substitution available to extensions so we need to provide our own mechanism

[plemarquand]

I'd given this a little bit of thought in the past. I've put up a patch for cursory support of common substitutions for settings I thought made sense.

Let me know what you think if the supported substitutions for the supported settings.

[award999]

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