[mlir][lsp] Register all extensions and TestDynDialect in main.

The main function of the LSP server needs to load the dialects and
similar that the server should be able to understand. When extensions
where introduced, the loading of the extensions was apparently not added
to its main functions, so ops from extensions were previously not
recognized by the server. This patch registers all extensions through
the existing convenience function, and also registers the
TestDynDialect, which `mlir-opt`s main function also registers.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D159091