Add the \markdownImportSnippets LaTeX command

[Witiko]

Suppose the jdoe/verylongpackagename/lists LaTeX theme defines the arabic, roman, and alphabetic setup snippets. If we want to access these snippets by their unqualified names, we must import them:

\markdownSetupSnippet{arabic}{snippet=jdoe/verylongpackagename/lists/arabic}
\markdownSetupSnippet{roman}{snippet=jdoe/verylongpackagename/lists/roman}
\markdownSetupSnippet{alphabetic}{snippet=jdoe/verylongpackagename/lists/alphabetic}

This is quite long and prone to typos. We may want to provide syntactic sugar:

\markdownImportSnippets{jdoe/verylongpackagename/lists}{arabic, roman, alphabetic}

[Witiko]

Some LaTeX themes may define snippets with conflicting unqualified names:

\markdownSetupSnippet{arabic1}{snippet=jdoe/verylongpackagename/lists/arabic}
\markdownSetupSnippet{arabic2}{snippet=jdoe/anotherlongpackagename/lists/arabic}
\markdownSetupSnippet{roman}{snippet=jdoe/verylongpackagename/lists/roman}
\markdownSetupSnippet{alphabetic}{snippet=jdoe/verylongpackagename/lists/alphabetic}

Our syntactic sugar may tackle this using the as operator:

\markdownImportSnippets{jdoe/verylongpackagename/lists}{arabic as arabic1, roman, alphabetic}
\markdownImportSnippets{jdoe/anotherlongpackagename/lists}{arabic as arabic2}

[Witiko]

Here is an alternative syntax that does not require a separate command:

\markdownSetup{
    importSnippets = {
        jdoe/longpackagename/lists = {
            arabic as arabic1,
            roman,
            alphabetic,
        }, jdoe/anotherlongpackagename/lists = {
            arabic as arabic2,
        },
    },
}

[Witiko]

Here is an alternative syntax that combines loading themes and snippets under a single interface, deprecates the LaTeX theme option.

Here is the current way you would load themes and import snippets from them:

\markdownSetup{theme=jdoe/verylongpackagename/lists}
\markdownSetupSnippet{arabic1}{snippet=jdoe/verylongpackagename/lists/arabic}
\markdownSetupSnippet{roman}{snippet=jdoe/verylongpackagename/lists/roman}
\markdownSetupSnippet{alphabetic}{snippet=jdoe/verylongpackagename/lists/alphabetic}
\markdownSetup{theme=jdoe/anotherlongpackagename/lists}
\markdownSetupSnippet{arabic2}{snippet=jdoe/anotherlongpackagename/lists/arabic}
\markdownSetup{theme=jdoe/yetanotherlongpackagename}

Here is the alternative syntax that gives the same result:

\markdownSetup{
    import = {
        jdoe/longpackagename/lists = {
            arabic as arabic1,
            roman,
            alphabetic,
        },
        jdoe/anotherlongpackagename/lists = {
            arabic as arabic2,
        },
        jdoe/yetanotherlongpackagename,
    },
}