Added reactive and viewable to init

[hoxbro]

Fixes #3135

I wanted to add all missing files to __init__ but got ImportError: cannot import name 'panel_extension' from partially initialized module 'panel.config' (most likely due to a circular import) (/home/shh/Development/holoviz/panel/panel/config.py)or similar. So settled with adding the files for the two import I miss the most pn.viewable.Viewer and pn.reactive.ReactiveHTML.

I used this code to be able to find missing imports:

import re
from pathlib import Path

def find_missing_import(file):
    file = Path(file)
    matches = re.findall(r"\w+", file.read_text())

    missing = []
    for f in sorted(file.parent.glob("*.py")):
        f = f.name[:-3]
        if f[0] != "_" and f not in matches:
            missing.append(f)

    return missing


files = Path("panel").rglob("__init__.py")
for file in files:
    if "tests" in str(file):
        continue
    
    missing = find_missing_import(file)
    if missing: 
        print(file, "->", ", ".join(missing))

Which outputs:

panel/__init__.py -> auth, compiler, util
panel/template/fast/__init__.py -> base, theme
panel/layout/__init__.py -> gridstack
panel/pane/vtk/__init__.py -> enums, synchronizable_deserializer, synchronizable_serializer
panel/io/__init__.py -> admin, datamodel, jupyter_server_extension, location, notifications, reload, rest, save
panel/models/__init__.py -> ace, comm_manager, deckgl, echarts, enums, katex, mathjax, perspective, plotly, quill, speech_to_text, tabulator, terminal, text_to_speech, vega, vtk

[codecov]

Codecov Report

Merging #3157 (c2196ea) into master (5bd9fc8) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3157      +/-   ##
==========================================
- Coverage   82.99%   82.97%   -0.03%     
==========================================
  Files         192      192              
  Lines       25482    25500      +18     
==========================================
+ Hits        21150    21159       +9     
- Misses       4332     4341       +9     

Impacted Files	Coverage Δ
panel/__init__.py	100.00% <100.00%> (ø)
panel/tests/command/test_serve.py	82.50% <0.00%> (-2.50%)	⬇️
panel/pane/markup.py	96.17% <0.00%> (-1.05%)	⬇️
panel/widgets/input.py	96.29% <0.00%> (-0.46%)	⬇️
panel/widgets/tables.py	84.76% <0.00%> (-0.22%)	⬇️
panel/viewable.py	70.44% <0.00%> (ø)
panel/io/save.py	62.60% <0.00%> (+0.32%)	⬆️

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