Parent issue: #19604

Description

Add Checkstyle checks for all rules under §4.3 - Method Names of the OpenJDK Java Style Guidelines v6.

Method Names
Method names should typically be verbs or other descriptions of actions.
Use mixed case with the first letter in lower case.

Rules covered

From §4.3 - Method Names:

1. Method name must start with a lowercase letter.
2. Method name must not contain _.

Checks to add to openjdk_checks.xml

TreeWalker-level:

  <module name="MethodName">
  <property name="format" value="^[a-z][^_]*$"/>
</module>

Progress tracker