Starting from version 1.13, execute commands cease to function when a slash is utilized in the subcommand

Prior to version 1.13, when employing the /execute command, you had the choice to include a / for the subcommand. However, since the overhaul of the command syntax in version 1.13, the command ceases to function when a / is added. This change may be deliberate, yet it proves inconvenient when copying a command to append it as a sub-command for an execute command, as the removal of the / must be done manually.

Steps to reproduce

1. Start version 1.12.2
2. Spawn a pig
3. Use the following command to confirm the expected behavior in 1.12.2:

   /execute @e[type=minecraft:pig] ~ ~ ~ /say hi

4. Start the newest version
5. Run the following command:

   /execute as @e[type=minecraft:pig] run /say hi

6. Run the following command:

   /execute as @e[type=minecraft:pig] run say hi

Observed behavior: The command fails to execute if the say command begins with a /, unlike prior to version 1.13.
Expected behavior: The command should continue to function properly even if the say command, which serves as the subcommand, includes a /.