Add config providers; always load .vscode/launch.json

[mfussenegger]

• Introduces a dap.providers.configs table which plugins can extend to add additional sources for dynamic configuration generation

• Implements the two built-in configuration sources as providers

• .vscode/launch.json files are now loaded automatically by one of these providers. The entries from the file always display even if the type doesn't match the current filetype. This gets rid of the type_to_filetypes mapping requirements.

Closes:

• https://github.com/mfussenegger/nvim-dap/issues/1065
• https://github.com/mfussenegger/nvim-dap/issues/1087
• https://github.com/mfussenegger/nvim-dap/issues/1226

[mfussenegger]

I should add that the naming is not final and might change before the next release without warning