Allow to add watchpoints to variables for source debugging. For instance:
`breakpoint set variable var`
will pause WAMR execution when the address at var is written to.
Can also set read/write watchpoints by passing r/w flags. This will pause
execution when the address at var is read:
`watchpoint set variable -w read var`
Add two linked lists for read/write watchpoints. When the debug message
handler receives a watchpoint request, it adds/removes to one/both of these
lists. In the interpreter, when an address is read or stored to, check whether
the address is in these lists. If so, throw a sigtrap and suspend the process.
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