Warnings removal PR(s)

[rusimody]

This is the summary of the warnings which this PR removes

org-tree-slide.el:1:1: Warning: file has no ‘lexical-binding’ directive on its
    first line
    
org-tree-slide.el:216:10: Warning: in defcustom for
    ‘org-tree-slide-heading-level-1’: ‘list’ without arguments
... many more

org-tree-slide.el:377:21: Warning: ‘point-at-bol’ is an obsolete function (as
    of 29.1); use ‘line-beginning-position’ or ‘pos-bol’ instead.
... many more


In org-tree-slide-skip-comments-toggle:
org-tree-slide.el:519:13: Warning: assignment to free variable
    ‘org-tree-slide--skip-comments-mode’
... many more
org-tree-slide.el:605:9: Warning: defvar ‘org-tree-slide-startup’ docstring
    wider than 80 characters

[rusimody]

Hi Takaaki!

I had forgotten about this and it just crept back into my radar
Any thoughts?

Would it help if I gave the more trivially harmless changes in a separate PR from those that could at least potentially have some compatibility issues

Trivially harmless

1. defvar before use
2. Shortening too long docstrings
3. Remove unused variables (The definition of v in function org-tree-slide--last-tree-p)

Unlikely to cause issues but at least conceptually could cause some issues

1. lexical binding
2. updating obsolete functions
3. Change face definition from defcustom to defface [You could also keep the defcustom and then quote the usage to silence the warning]