Improve backtick regex matching for function names

[ncoop57]

This PR improves the regex detection for doclinks in nbdev. Specifically it now handles parenthesis being included in backticks:

md = """This is a link to `numpy.array` and to `get_config()` but not a link to `foobar`.
And not a link to <code>dict2nb</code>.

    This is not a link to `get_config`
"""
print(NbdevLookup('nbdev').linkify(md))

This is a link to [`numpy.array`](https://numpy.org/doc/stable/reference/generated/numpy.array.html#numpy.array) and to [`get_config()`](https://nbdev.fast.ai/api/config.html#get_config) but not a link to `foobar`.
And not a link to <code>dict2nb</code>.

    This is not a link to `get_config`

Additional tests were also added to check against edge cases.

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[ncoop57]

@jph00

[jph00]

Great stuff @ncoop57 :D