Mathml bug fix

[zmx0142857]

main changes:

• support mathml in playground
• support border by applying styles for matrices
• support verb command
• partly support multiline by usage of mtable

however, there remains some test cases to fix:

• limits command
• choice of correct pipe symbol in expression like (|a| + |b| + |c)
• and, a bug came from nearley grammar file: (|a-b|^2)

you can merge this PR if you think these bugs are minor!

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