If Outvoke was started with no VRChat log file present and then
VRChat was started with Outvoke still running, Outvoke could
terminate with an error.

This has been fixed.

how to repro:

---------------
1. While VRChat is not running, delete all VRChat log files

2. launch Outvoke as follows:

$ ./outvoke.rb -e 'hook "vrchat-001", /pickup/i'

3. start VRChat
---------------

This commit changes around "enabled?".

For all data sources, the default value of "enabled?" is now
false.

And if "enabled?" is false, neither before_each_event nor
after_each_event is executed anymore.

With these changes, VRChat log file will not be read unless
a Proc is registered by hook to the data source "vrchat-001".

For the time being, "sources" will remain by alias_method.
Eventually "sources" will be removed.

"pre_procs" is for the process before the execution of a Proc
registered by hook.

When overwriting e.body with pre_procs, it should be noted that
it will be executed as many times as the number of hooks.

"post_procs" treats the value returned by Proc registered by hook.

This new feature "post_procs" is related to 04be13b4 and 00b2fb7c.

This commit fixes problem with return value being ignored when
no block was passed to hook.

This commit removes a feature "ignore_list". Use new feature
"nodup" instead of "ignore_list".

With this change, any value returned by a Proc registered by hook
that is not true, false, nil, etc. will now be converted to a
string and output.

----
$ seq 1 3 | ./outvoke.rb -q -e 'hook("stdin") { _1.body.to_i * 2 }'
2
4
6
----

This commit makes the following changes:

  - add new class OutvokeProcExecContext
  - add new feature "to" method
  - change to use instance_exec in two places

These will allow you can write like following:

hook("every-sec", / ..:..:.[02468]/) { to "lo" }

Since loputs are already provided for the data source "lo", the above
code is equivalent to writing this:

hook("every-sec", / ..:..:.[02468]/) { loputs _1.body }

Using instance_exec allows you to write something like `to "lo"`.

OutvokeProcExecContext is a class provided only for instance_exec.

Ever since the initial release of outvoke.rb (35a76256), Proc#call
has simply been used to execute Proc objects registered by hook.
With this commit, for the first time, a change is being made here
so that instance_exec will be used.

Note that the impact of this has not been fully tested.

BTW, This commit is related to 7dba2139 and 9a253737.

In VRChat client version 2024.4.1 (Build 1533), the log file output
format has changed.

https://docs.vrchat.com/docs/vrchat-202441
The above page contains the following text:

> Output logs now include user IDs when players join or leave
> an instance.

So in this new sample code, user IDs are output in an easily
readable format.

On the whole, this new sample code works much like
samples/vrchat_join_log.rb.

In VRChat client version 2024.4.1 (Build 1533), the log file output
format has changed.

https://docs.vrchat.com/docs/vrchat-202441
The above page contains the following text:

> Output logs now include user IDs when players join or leave
> an instance.

Log entries containing "OnPlayerJoinComplete" seem to output the same
as before, so I would change this sample vrchat_join_log2.rb to use
/onplayerjoincomplete/i instead of /onplayerjoined/i.