New rule: math-missing-invisible-function-operator

[TimArborealis]

Suggested name: math-missing-invisible-function-operator

This rule should check for instances where the invisible function operator (⁡
) is missing:

1. Between a recognised function and its argument. Recognised function names should include

• sin
• cos
• tan
• ln
• lg
• log
• sinh
• cosh
• tanh
• exp

2. Between a commonly used function name and an open parenthesis. Commonly used function names should include (only lowercase)

• f
• g
• h

See examples of what should be flagged in test_issue_23.epub

@MartheGjelstad, thoughts? (created together with @AntonLMTM )