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/testdata/return/fail_expr_no_semi.carbon

@@ -10,7 +10,7 @@
 
 fn F() {
   return x
-// CHECK:STDERR: fail_expr_no_semi.carbon:[[@LINE+3]]:1: ERROR: `return` statements must end with a `;`.
+// CHECK:STDERR: fail_expr_no_semi.carbon:[[@LINE+3]]:1: error: `return` statements must end with a `;`
 // CHECK:STDERR: }
 // CHECK:STDERR: ^
 }

toolchain/parse/testdata/return/fail_no_semi.carbon

@@ -10,11 +10,11 @@
 
 fn F() {
   return
-// CHECK:STDERR: fail_no_semi.carbon:[[@LINE+7]]:1: ERROR: Expected expression.
+// CHECK:STDERR: fail_no_semi.carbon:[[@LINE+7]]:1: error: expected expression
 // CHECK:STDERR: }
 // CHECK:STDERR: ^
 // CHECK:STDERR:
-// CHECK:STDERR: fail_no_semi.carbon:[[@LINE+3]]:1: ERROR: `return` statements must end with a `;`.
+// CHECK:STDERR: fail_no_semi.carbon:[[@LINE+3]]:1: error: `return` statements must end with a `;`
 // CHECK:STDERR: }
 // CHECK:STDERR: ^
 }

toolchain/parse/testdata/return/fail_returned_no_var.carbon

@@ -9,7 +9,7 @@
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/return/fail_returned_no_var.carbon
 
 fn F() -> String {
-  // CHECK:STDERR: fail_returned_no_var.carbon:[[@LINE+3]]:12: ERROR: Expected `var` after `returned`.
+  // CHECK:STDERR: fail_returned_no_var.carbon:[[@LINE+3]]:12: error: expected `var` after `returned`
   // CHECK:STDERR:   returned fn G() -> i32;
   // CHECK:STDERR:            ^~
   returned fn G() -> i32;

toolchain/parse/testdata/return/fail_var_no_semi.carbon

@@ -10,7 +10,7 @@
 
 fn F() {
   return var
-// CHECK:STDERR: fail_var_no_semi.carbon:[[@LINE+3]]:1: ERROR: `return` statements must end with a `;`.
+// CHECK:STDERR: fail_var_no_semi.carbon:[[@LINE+3]]:1: error: `return` statements must end with a `;`
 // CHECK:STDERR: }
 // CHECK:STDERR: ^
 }