Replace BitsSignedB{32,64}Le with better BitsSignedShiftB{32,64}Le

Note: BitsSignedB{32,64}Le were removed in the previous commit 43d8b74

According to my tests, new formats BitsSignedShiftB{32,64}Le have better
detection capabilities than their predecessors. Old tests only detected
kaitai-io/kaitai_struct_javascript_runtime@1f8dd95a,
but did not catch the bug when I intentionally change the appropriate
unsigned right shift `>>>` to signed `>>` in the new `readBitsIntLe` impl
introduced in kaitai-io/kaitai_struct#949. Tests
added in this commit reveal both of these errors.

formats/bits_signed_shift_b32_le.ksy

@@ -0,0 +1,11 @@
+# Make sure that languages with 32-bit operators (JavaScript) use *unsigned*
+# (aka zero-fill) right shift, not signed (sign-propagating) right shift. This
+# test will fail if a signed shift is used.
+meta:
+  id: bits_signed_shift_b32_le
+  bit-endian: le
+seq:
+  - id: a
+    type: b31
+  - id: b
+    type: b9

formats/bits_signed_shift_b64_le.ksy

@@ -0,0 +1,11 @@
+# Make sure that languages with 64-bit operators (Java, PHP) use *unsigned* (aka
+# zero-fill) right shift, not signed (sign-propagating) right shift. This test
+# will fail if a signed shift is used.
+meta:
+  id: bits_signed_shift_b64_le
+  bit-endian: le
+seq:
+  - id: a
+    type: b63
+  - id: b
+    type: b9

spec/ks/bits_signed_shift_b32_le.kst

@@ -0,0 +1,7 @@
+id: bits_signed_shift_b32_le
+data: bits_signed_shift_b32_le.bin
+asserts:
+  - actual: a
+    expected: 0
+  - actual: b
+    expected: 0xff

spec/ks/bits_signed_shift_b64_le.kst

@@ -0,0 +1,7 @@
+id: bits_signed_shift_b64_le
+data: bits_signed_shift_b64_le.bin
+asserts:
+  - actual: a
+    expected: 0
+  - actual: b
+    expected: 0xff