feat(lsp): add supports_method to nio.lsp.Client

[rcasia]

The straight way to check whether a method is supported is via supports_method.

News 0.10.0:

• Dynamic registration of LSP capabilities. An implication of this change is
  that checking a client's server_capabilities is no longer a sufficient
  indicator to see if a server supports a feature. Instead use
  client.supports_method(<method>). It considers both the dynamic
  capabilities and static server_capabilities.

For example, nvim-jdtls does not have definitionProvider in server capabilities. See mfussenegger/nvim-jdtls#589 (comment).

Relates to nvim-neotest/neotest#425

[rcarriga]

Thanks for the PR!