perf: Lazily require adapter mappings

[Natr1x]

Implementation of the performance improvement in #70 that should not cause #73 #74 which required the previous pull request #75 to need to be reversed.

Why?

The change in #70 returned errors whenever trying to get an adapter which was not mapped instead of just returning nil.

What

The fix is simply to return nil for unmapped adapters. (Something which I thought of but then apparently did not do for some reason..)

Testing Done (and which can be done)

I have tested with installing and running java-debug-adapter #73 which caused an error because it is not mapped in mason-nvim-dap.mappings.adapters and elixir-ls #74 which will try to get an adapter for 'elixir' when the handler is set up, which will is also not a mapping that exists.

I am not regularly using these these adapters however so I would probably be good if someone who has encountered the problems in #73 and #74 tests this before any merging is done.

[jay-babu]

Thanks for all the help! merging attempt 2

[y1rn]

lua/mason-nvim-dap/mappings/adapters/init.lua

local adapter, ok = pcall(require, 'mason-nvim-dap.mappings.adapters.' .. key)

should be

local ok, adapter = pcall(require, 'mason-nvim-dap.mappings.adapters.' .. key)