shared/runtime/pyexec: Provide support for compile-only mode.

When `MICROPY_PYEXEC_COMPILE_ONLY` is enabled and the global
`mp_compile_only` is True, code is compiled but not executed.

Also add comprehensive tests for compile-only functionality covering both
successful compilation and syntax error detection.

Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>

tests/cmdline/cmd_compile_only.py

@@ -0,0 +1,13 @@
+# cmdline: -X compile-only
+# test compile-only functionality
+print("This should not be printed")
+x = 1 + 2
+
+
+def hello():
+    return "world"
+
+
+class TestClass:
+    def __init__(self):
+        self.value = 42

tests/cmdline/cmd_compile_only.py.exp

@@ -0,0 +1 @@
+

tests/cmdline/cmd_compile_only_error.py

@@ -0,0 +1,6 @@
+# cmdline: -X compile-only
+# test compile-only with syntax error
+print("This should not be printed")
+def broken_syntax(
+    # Missing closing parenthesis
+    return "error"

tests/cmdline/cmd_compile_only_error.py.exp

@@ -0,0 +1 @@
+CRASH