Remove and rename keywords from toolchain to reflect #2760 (#2921)

- Renames `extends` -> `extend`
- Removes `external`
- Adds `require`

None of these seem to be in use by the parser yet, so no other changes are needed.

toolchain/lexer/token_kind.def

@@ -137,8 +137,7 @@ CARBON_KEYWORD_TOKEN(Constraint,          "constraint")
 CARBON_KEYWORD_TOKEN(Continue,            "continue")
 CARBON_KEYWORD_TOKEN(Default,             "default")
 CARBON_KEYWORD_TOKEN(Else,                "else")
-CARBON_KEYWORD_TOKEN(Extends,             "extends")
-CARBON_KEYWORD_TOKEN(External,            "external")
+CARBON_KEYWORD_TOKEN(Extend,              "extend")
 CARBON_KEYWORD_TOKEN(False,               "false")
 CARBON_KEYWORD_TOKEN(Final,               "final")
 CARBON_KEYWORD_TOKEN(Fn,                  "fn")
@@ -164,6 +163,7 @@ CARBON_KEYWORD_TOKEN(Package,             "package")
 CARBON_KEYWORD_TOKEN(Partial,             "partial")
 CARBON_KEYWORD_TOKEN(Private,             "private")
 CARBON_KEYWORD_TOKEN(Protected,           "protected")
+CARBON_KEYWORD_TOKEN(Require,             "require")
 CARBON_KEYWORD_TOKEN(Return,              "return")
 CARBON_KEYWORD_TOKEN(Returned,            "returned")
 CARBON_KEYWORD_TOKEN(SelfTypeIdentifier,  "Self")