fix(caddyfile): Expand imported snippets within named routes at parse time

[glossawy]

Fixes #6431

Handles imports and any other potential current or future parse-time constructs by taking block tokens for a named route block and running those tokens through the entire parsing system itself, like a nested caddyfile with any context, defined snippets, and metadata inherited from its "parent" parser.

The proposal here is just to create a new "child" parser with the necessary cloned data, parsing the named route block with that parser, and then yielding back the resulting parsed tokens and any necessary changes to e.g. the import graph.

This way any changes to the parsing in general will apply to named routes as well and otherwise leaves the interpretation of tokens after parsing the same since the structure for named routes remains the same as before: a normal server block with a known invoke-able name.

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