feat(NumberTheory/LSeries/Dirichlet): new file, material on specific L-series

[MichaelStollBayreuth]

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This PR adds a new file NumberTheory.LSeries.Dirichlet that contains results on L-series of specific functions:

• the Möbius function
• Dirichlet characters, with the constant function 1 as a special case
• the arithmetic function ζ (which has the same L-series as the constant function 1)
• the von Mangoldt function and its twists by Dirichlet characters

It also adds (L-series of zero and of the indicator function of {1}) and removes (convergence of the L-series of the constant function 1 / of ζ; this is moved to the new file) some material to/from NumberTheory.LSeries.Basic.

See this thread on Zulip.

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