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_binding_pattern.cpp

@@ -28,8 +28,7 @@ auto HandleBindingPattern(Context& context) -> void {
   auto on_error = [&](llvm::StringLiteral expected) {
     if (!state.has_error) {
       CARBON_DIAGNOSTIC(ExpectedBindingPattern, Error,
-                        "Expected {0} in binding pattern.",
-                        llvm::StringLiteral);
+                        "expected {0} in binding pattern", llvm::StringLiteral);
       context.emitter().Emit(*context.position(), ExpectedBindingPattern,
                              expected);
       state.has_error = true;