Add matchEntityType(String) method to EntityType class. Adds BUKKIT-5777

##### The Issue:

The EntityType enum, while mostly identical to the Material enum, lacks a method corresponding to Material#matchMaterial(String).

##### Justification:

In the interest of consistency and ease of use, a corresponding method is beneficial to the API.

##### PR Breakdown:

Creates a method in EntityType called matchEntityType(String), which formats the argument appropriately and matches it to a corresponding value of the enum. Returns null if none is found.

##### Testing Results and Materials:

See EntityTypeTest.java.

##### JIRA Ticket:

Bukkit-5777 - https://bukkit.atlassian.net/browse/BUKKIT-5777?filter=10004
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