Allow simplified input for attribute values

[MrSpoocy]

Description

Related to the https://commonmark.thephpleague.com/2.3/extensions/attributes/ extension, I would like to make an adjustment. In order to make it easier for the users, I would be a bit more generous when entering values for atttributes (can be activated via configuration).

For all key-value values, where the value only consists of the following characters A-Za-z0-9-_. the user does not need to write quotation marks (they are set in the HTML anyway).

It is only an improvement for the operability. What is your opinion?
I would make this adjustment as part of the change in #942.

Example

[Goolge can help you](https://www.google.de){target=_blank} -> <a href="https://www.google.de" target="_blank">Goolge can help you</a>

### Headline {class=big} -> <h3 class="big">Headline</h3>

Total crazy{class=cms.highlight} -> <p class="cms.highlight">Total crazy</p>

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No response

[colinodell]

Looking at one of our test cases it seems like this might already be supported:

commonmark/tests/functional/Extension/Attributes/data/table_attributes.md

Lines 1 to 4 in 32891d1

	h | h
	----------|---
	{.head} c1 |{.head} c2
	{summary="Table summary" .class1 style="color:red" border=3 width="50%" frame=lhs rules=cols cellspacing=2em cellpadding=4px}

Would you be open to adding some more test cases to confirm whether this works as expected? We can then commit those new test cases if they pass or explore options (like your proposal) if they don't currently work.

[MrSpoocy]

Then maybe it's a bug I found. I will extend the tests tomorrow morning. What I can already say, with me the following did not go (the point at the end provides for problems):

This is
a test [Google](https://www.google.de){target=_blank}.

Second line