Improve entity scopes, take two

[winstliu]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

Redo of #98, this time taking into account that entities in attributes behave differently. I tried to follow the spec as closely as possible.

Alternate Designs

• The list of valid entities could be included. I'd rather not do that though.
• Single-line matches before the begin/end pattern removes the potential for runaway rules, but means that all entities will appear as invalid before they are finished.

Benefits

Realize you're writing an entity before you finish doing so. This also benefits autocomplete-html.

Possible Drawbacks

There may still be room for regressions that I didn't take into account.

Applicable Issues

See #98, #141, #142.

/cc @DavidePastore, @JonathanWolfe - see anything that could potentially break with this version?

[DavidePastore]

This looks ok for me 👍 Maybe you could add a test with a new line after the "&" character, and check that it works as expected. Thanks for including me in this discussion. 😃

[winstliu]

Sorry for the late response @DavidePastore. Can you elaborate on what the expected behavior should be, and would you like to see a test for both text and attribute entities, or just text?

[JonathanWolfe]

LGTM

[DavidePastore]

@50Wliu Sorry for the delay. I'd like to see a test for both text and attribute entities (for example in the placeholder attribute). For both it should be possibile to have a & character and then a new line. Anyway maybe it could be a redundant test.

[winstliu]

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