refactor!: extract mason-lspconfig to separate plugin (#109)

The rationale behind this is to make boundaries clearer as mason.nvim
has no direct relation with lspconfig per se.

Also, hopefully, by having it as a separate package like this would
encourage more people to write similar extensions (think mason-dap and
mason-null-ls). Ideally such extensions wouldn't be required at all, but
there are definitely gaps to fill as of today.

From now on you'll need to add `williamboman/mason-lspconfig.nvim` as
a plugin if you want to use the `mason-lspconfig` extension:

```lua
use {
    { "williamboman/mason.nvim", branch = "alpha" },
    "williamboman/mason-lspconfig.nvim",
    "neovim/nvim-lspconfig",
}
```

```lua
Plug "williamboman/mason.nvim", { 'branch': 'alpha' }
Plug "williamboman/mason-lspconfig.nvim"
Plug "neovim/nvim-lspconfig"
```

1 files changed, 0 insertions, 5 deletions

diff --git a/lua/mason-lspconfig/server_configurations/rescriptls/init.lua b/lua/mason-lspconfig/server_configurations/rescriptls/init.lua
deleted file mode 100644
index 2f55cc30..00000000
--- a/lua/mason-lspconfig/server_configurations/rescriptls/init.lua
+++ /dev/null
@@ -1,5 +0,0 @@
-return function()
-    return {
-        cmd = { "rescript-lsp", "--stdio" },
-    }
-end