Add framework for the extern keyword. (#3755)

This doesn't add full support. I'm separating it out to make the effects
of the modifier changes clearer for review. I'm restructuring a little
with the expectation that we'll have some more categories of modifier
keywords in the future (similar to `extern`, these may not be in a "set"
such as access), and thus easily scaling up to a few more would be
useful.

toolchain/parse/testdata/function/declaration/basic.carbon

@@ -5,6 +5,7 @@
 // AUTOUPDATE
 
 fn F();
+extern fn G();
 
 // CHECK:STDOUT: - filename: basic.carbon
 // CHECK:STDOUT:   parse_tree: [
@@ -14,5 +15,11 @@ fn F();
 // CHECK:STDOUT:         {kind: 'TuplePatternStart', text: '('},
 // CHECK:STDOUT:       {kind: 'TuplePattern', text: ')', subtree_size: 2},
 // CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 5},
+// CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:       {kind: 'ExternModifier', text: 'extern'},
+// CHECK:STDOUT:       {kind: 'IdentifierName', text: 'G'},
+// CHECK:STDOUT:         {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:       {kind: 'TuplePattern', text: ')', subtree_size: 2},
+// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 6},
 // CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT:   ]