refactor(filetype)!: allow vim.filetype.match to use different strategies (#18895)

This enables vim.filetype.match to match based on a buffer (most
accurate) or simply a filename or file contents, which are less accurate
but may still be useful for some scenarios.

When matching based on a buffer, the buffer's name and contents are both
used to do full filetype matching. When using a filename, if the file
exists the file is loaded into a buffer and full filetype detection is
performed. If the file does not exist then filetype matching is only
performed against the filename itself. Content-based matching does the
equivalent of scripts.vim, and matches solely based on file contents
without any information from the name of the file itself (e.g. for
shebangs).

BREAKING CHANGE: use `vim.filetype.match({buf = bufnr})` instead 
of `vim.filetype.match(name, bufnr)`

test/functional/lua/filetype_spec.lua

@@ -3,6 +3,7 @@ local exec_lua = helpers.exec_lua
 local eq = helpers.eq
 local clear = helpers.clear
 local pathroot = helpers.pathroot
+local command = helpers.command
 
 local root = pathroot()
 
@@ -23,7 +24,7 @@ describe('vim.filetype', function()
           rs = 'radicalscript',
         },
       })
-      return vim.filetype.match('main.rs')
+      return vim.filetype.match({ filename = 'main.rs' })
     ]])
   end)
 
@@ -37,7 +38,7 @@ describe('vim.filetype', function()
           ['main.rs'] = 'somethingelse',
         },
       })
-      return vim.filetype.match('main.rs')
+      return vim.filetype.match({ filename = 'main.rs' })
     ]])
   end)
 
@@ -48,7 +49,7 @@ describe('vim.filetype', function()
           ['s_O_m_e_F_i_l_e'] = 'nim',
         },
       })
-      return vim.filetype.match('s_O_m_e_F_i_l_e')
+      return vim.filetype.match({ filename = 's_O_m_e_F_i_l_e' })
     ]])
 
     eq('dosini', exec_lua([[
@@ -59,7 +60,7 @@ describe('vim.filetype', function()
           [root .. '/.config/fun/config'] = 'dosini',
         },
       })
-      return vim.filetype.match(root .. '/.config/fun/config')
+      return vim.filetype.match({ filename = root .. '/.config/fun/config' })
     ]], root))
   end)
 
@@ -72,11 +73,13 @@ describe('vim.filetype', function()
           ['~/blog/.*%.txt'] = 'markdown',
         }
       })
-      return vim.filetype.match('~/blog/why_neovim_is_awesome.txt')
+      return vim.filetype.match({ filename = '~/blog/why_neovim_is_awesome.txt' })
     ]], root))
   end)
 
   it('works with functions', function()
+    command('new')
+    command('file relevant_to_me')
     eq('foss', exec_lua [[
       vim.filetype.add({
         pattern = {
@@ -87,7 +90,7 @@ describe('vim.filetype', function()
           end,
         }
       })
-      return vim.filetype.match('relevant_to_me')
+      return vim.filetype.match({ buf = 0 })
     ]])
   end)
 end)