Add ability to Skip Param<Ref|Function|Method> updates

[philippjfr]

This PR is the counter-part to a change in Param introduced in holoviz/param#908. Specifically we add support for handling param.Skip exceptions. This allows writing reactive callbacks that skip events if certain conditions aren't met. This can be useful in scenarios where you want to skip updates for invalid parameter combinations or if you want to wait on a button click or some other event to actually re-evaluate the function or method.

An example is submission form:

name = pn.widgets.TextInput()
age = pn.widgets.FloatInput()
button = pn.widgets.Button(name='Submit!')

def submit_fn(name, age, submit):
    if not submit:
        raise Skip()
    return 'Submitted user {name}  of {age=}.'

pn.bind(submit_fn, name, age, button)

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 71.64%. Comparing base (93b8d95) to head (1c0fdd4).

Files	Patch %	Lines
panel/param.py	86.36%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6396      +/-   ##
==========================================
+ Coverage   71.61%   71.64%   +0.02%     
==========================================
  Files         310      310              
  Lines       45841    45894      +53     
==========================================
+ Hits        32831    32882      +51     
- Misses      13010    13012       +2     

Flag	Coverage Δ
unitexamples-tests	71.64% <94.73%> (+0.02%)	⬆️

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