Consolidate loadable extension features

[mlafeldt]

Having two separate features was confusing:

• vtab-loadable - macros only
• loadable-extension - linking changes only

Users needed both for working extensions. Using vtab-loadable alone produced non-functional extensions (DuckDB linked statically into cdylib → two DuckDB instances when loaded → broken).

Before:

duckdb = { features = ["vtab-loadable", "loadable-extension"] }

After:

duckdb = { features = ["loadable-extension"] }

vtab-loadable is preserved as a deprecated alias pointing to loadable-extension. Existing Cargo.toml files continue to work.

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What does this mean for https://github.com/duckdb/extension-template-rs?

After the next release, we can change

[dependencies]
duckdb = { version = "=1.4.3", features = ["vtab-loadable"] }
duckdb-loadable-macros = "=0.1.13"
libduckdb-sys = { version = "=1.4.3", features = ["loadable-extension"] }

to

duckdb = { version = "=1.4.4", features = ["loadable-extension"] }

(though the old way will still work)