Add `read_snapshot!` and `read_binary_snapshot!` macros (experimental)

[max-sixty]

for https://github.com/mitsuhiko/insta/issues/353

I think it's somewhat useful. It doesn't allow reading from a different function. As a downside: it adds to the API surface...

From Claude (which also helped with the code):

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These new experimental macros allow programmatic reading of snapshot contents, returning String for text snapshots and Vec<u8> for binary snapshots.

The macros resolve snapshot paths using the same logic as assert_snapshot! and assert_binary_snapshot!, respecting settings like snapshot_path, snapshot_suffix, and prepend_module_to_snapshot.

Includes comprehensive functional tests covering various scenarios such as:

• Reading YAML, JSON, and plain text snapshots
• Handling nonexistent snapshots
• Auto-generated snapshot names
• Custom snapshot_path
• Binary snapshots with read_binary_snapshot!
• snapshot_suffix
• prepend_module_to_snapshot => false
• Nested modules
• Combined settings
• Error cases for reading text as binary and vice-versa.