Include scanner.c in rust binding build

[trevnels]

Previously, building the rust binding would fail with the following error, since scanner.c was not included in the build:

/usr/bin/ld: undefined reference to `tree_sitter_typst_external_scanner_create'
/usr/bin/ld: undefined reference to `tree_sitter_typst_external_scanner_destroy'
/usr/bin/ld: undefined reference to `tree_sitter_typst_external_scanner_scan'
/usr/bin/ld: undefined reference to `tree_sitter_typst_external_scanner_serialize'
/usr/bin/ld: undefined reference to `tree_sitter_typst_external_scanner_deserialize'

This PR includes it so that the rust binding builds successfully.