[FEATURE]: Default argument values in custom slash commands and (opencode expand).

[ariane-emory]

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Let's add a new syntax that can be used in custom slash commands to provide default values for arguments.

An argument with a default value: ${2:foobar}

Above is an example of a slash command containing an argument with the default value. If the value of the second argument is not an empty string (or a string consisting solely of whitespace), its value is used; otherwise, the value foobar is used.

This feature should build upon the work done in the refactor/shared-substitute branch and be integrated into the shared substitution function added by the aforementioned branch.

This feature should be independent of, but fully compatible with the following branch's features: feat/argument-range-syntax and feat/opencode-expand: the feature should work properly if these branches are not also merged, but if they are also merged then it should work in conjunction with them as well: it should be possible to use this in files processed by an opencode expand command (if also merged) or in conjunction with the argument range syntax (if also merged, examples: ${2..3:foo bar baz}, ${3..:a default string}, ${..:another default string}).