Update basic diagnostic capitalization/punctuation (#4328)

This is a primarily automated change:

- Search & replace for capitalization
-
`(CARBON_DIAGNOSTIC\((?:\n\s+)?\w+,(?:\n\s+)?\s\w+,(?:\n\s+)?\s")([A-Z])`
    - `$1\L$2`
- Search & replace for period
-
`(CARBON_DIAGNOSTIC\((?:\n\s+)?\w+,(?:\n\s+)?\s\w+,(?:\n\s+)?\s"(?:[^)]|\n)+)\.("[,)])`
    - `$1$2`
- Limited search & replace for `ERROR: ` -> `error: ` in streamed things
- Leaving a TODO for command_line because there's more cleanup that can
be done there
- Modify diagnostic_consumer.cpp
    - ERROR -> error
    - WARNING -> warning

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/parse/handle_statement.cpp

@@ -78,7 +78,7 @@ static auto HandleStatementKeywordFinish(Context& context, NodeKind node_kind)
   auto semi = context.ConsumeIf(Lex::TokenKind::Semi);
   if (!semi) {
     CARBON_DIAGNOSTIC(ExpectedStatementSemi, Error,
-                      "`{0}` statements must end with a `;`.", Lex::TokenKind);
+                      "`{0}` statements must end with a `;`", Lex::TokenKind);
     context.emitter().Emit(*context.position(), ExpectedStatementSemi,
                            context.tokens().GetKind(state.token));
     state.has_error = true;
@@ -112,7 +112,7 @@ auto HandleStatementForHeader(Context& context) -> void {
     context.AddLeafNode(NodeKind::VariableIntroducer, context.Consume());
   } else {
     CARBON_DIAGNOSTIC(ExpectedVariableDecl, Error,
-                      "Expected `var` declaration.");
+                      "expected `var` declaration");
     context.emitter().Emit(*context.position(), ExpectedVariableDecl);
 
     if (auto next_in = context.FindNextOf({Lex::TokenKind::In})) {