Rename executable_semantics to explorer (#1188)

Change generated with:

```
#!/usr/bin/bash -eux

# Helper script for renaming pending work.
# Run from the repo root.

# Rename executable_semantics in code.
sed -i 's/executable_semantics/explorer/g' \
  $(git grep -l 'executable_semantics' . | grep -v proposals)
sed -i 's/executable semantics/explorer/g' \
  $(git grep -l 'executable semantics' . | grep -v proposals)
sed -i 's/Executable semantics/Explorer/g' \
  $(git grep -l 'Executable semantics' . |  grep -v proposals)
sed -i 's/Executable Semantics/Explorer/g' \
  $(git grep -l 'Executable Semantics' . |  grep -v proposals)
sed -i 's/EXECUTABLE_SEMANTICS/EXPLORER/g' \
  $(git grep -l 'EXECUTABLE_SEMANTICS' . | grep -v proposals)
sed -i 's/ExecutableSemantics/Explorer/g' \
  $(git grep -l 'ExecutableSemantics' . | grep -v proposals)
sed -i 's/executable-semantics/explorer/g' \
  $(git grep -l 'executable-semantics' . | grep -v proposals)

# This is only needed for the initial move.
mv executable_semantics explorer
mv explorer/fuzzing/executable_semantics_fuzzer.cpp explorer/fuzzing/explorer_fuzzer.cpp
```

Verified with `bazel test ...`

explorer/syntax/unimplemented_example_test.cpp

@@ -0,0 +1,36 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include "explorer/ast/ast_test_matchers.h"
+#include "explorer/syntax/parse.h"
+#include "explorer/syntax/parse_test_matchers.h"
+
+namespace Carbon::Testing {
+namespace {
+
+using ::testing::ElementsAre;
+
+TEST(UnimplementedExampleTest, VerifyPrecedence) {
+  static constexpr std::string_view Program = R"(
+    package ExplorerTest api;
+    fn Main() -> i32 {
+      return 1 __unimplemented_example_infix 2 == 3;
+    }
+  )";
+  Arena arena;
+  EXPECT_THAT(ParseFromString(&arena, "dummy.carbon", Program, false),
+              ParsedAs(ASTDeclarations(
+                  ElementsAre(MatchesFunctionDeclaration().WithBody(
+                      BlockContentsAre(ElementsAre(MatchesReturn(MatchesEq(
+                          MatchesUnimplementedExpression(
+                              "ExampleInfix", ElementsAre(MatchesLiteral(1),
+                                                          MatchesLiteral(2))),
+                          MatchesLiteral(3))))))))));
+}
+
+}  // namespace
+}  // namespace Carbon::Testing