Add API to access watchers

[maximlt]

Following @philippjfr's comment in #766 (comment), we have identified the need to provide an API to access watchers.

More precisely, it seems like we do not have an API to access watchers on an instance, which are currently available through the private attribute _param_watchers:

import param

class P(param.Parameterized):
    x = param.Parameter()
    
    @param.depends('x', watch=True)
    def d(self): pass

p = P()
print(p._param_watchers)
# {'x': {'value': [Watcher(inst=P(name='P00005', x=None), cls=<class '__main__.P'>, fn=<function _m_caller.<locals>.caller at 0x118fa5c10>, mode='args', onlychanged=True, parameter_names=('x',), what='value', queued=False, precedence=-1)]}}
print(p.param['x'].watchers)
# {}

There is a sort of public API to access the watchers set on class Parameters:

import param

class P(param.Parameterized):
    x = param.Parameter()

P.param.watch(lambda e: print(e), 'x')
print(P.param['x'].watchers)
# {'value': [Watcher(inst=None, cls=<class '__main__.P'>, fn=<function <lambda> at 0x103f3b1f0>, mode='args', onlychanged=True, parameter_names=('x',), what='value', queued=False, precedence=0)]}

There is the public .param.watchers that under the hood returns the watchers queue defined on _parameters_state, that holds transient data:

import param

class P(param.Parameterized):
    x = param.Parameter()
    
    @param.depends('x', watch=True)
    def d(self): pass

p = P()
p._param_watchers

print('start', p.param.watchers)
with param.parameterized.batch_call_watchers(p):
    print('   start cm', p.param.watchers)
    p.x = 1
    p.x = 2
    p.x = 3
    print('   end cm', p.param.watchers)
print('end', p.param.watchers)

Output:

start []
   start cm []
   end cm [Watcher(inst=P(name='P00005', x=3), cls=<class '__main__.P'>, fn=<function _m_caller.<locals>.caller at 0x118fa5c10>, mode='args', onlychanged=True, parameter_names=('x',), what='value', queued=False, precedence=-1)]
end []

---

Among those _param_watchers is likely the one the most required by users. It's accessed in Panel and Lumen, I've found one other usage on Github https://github.com/grepler/sqlmodel-panel-poc/blob/bf8b3ab1b3e31476f6fcbb451a7dc78fb555d29d/helpers.py#L215.

[maximlt]

Do you have any opinion on this @philippjfr ? It seems it's unfortunate that .param.watchers is already used, as it seems more like an internal API made public.