fix(ui)!: change filetype from "mason.nvim" to "mason"

Dots in 'filetype' are used for composite filetypes. By using
`mason.nvim` as filetype, we're effectively assigning to filetypes:
`mason` and `nvim`.

lua/mason/ui/instance.lua

@@ -110,7 +110,7 @@ local function remove(list, item)
     return list
 end
 
-local window = display.new_view_only_win("mason.nvim", "mason.nvim")
+local window = display.new_view_only_win("mason.nvim", "mason")
 local packages = _.sort_by(_.prop "name", registry.get_all_packages())
 
 window.view(

tests/mason/api/command_spec.lua

@@ -15,7 +15,7 @@ describe(":Mason", function()
             a.scheduler()
             local win = vim.api.nvim_get_current_win()
             local buf = vim.api.nvim_win_get_buf(win)
-            assert.equals("mason.nvim", vim.api.nvim_buf_get_option(buf, "filetype"))
+            assert.equals("mason", vim.api.nvim_buf_get_option(buf, "filetype"))
         end)
     )
 end)
@@ -44,7 +44,7 @@ describe(":MasonInstall", function()
             api.MasonInstall { "dummy" }
             local win = vim.api.nvim_get_current_win()
             local buf = vim.api.nvim_win_get_buf(win)
-            assert.equals("mason.nvim", vim.api.nvim_buf_get_option(buf, "filetype"))
+            assert.equals("mason", vim.api.nvim_buf_get_option(buf, "filetype"))
         end)
     )
 end)