[feature] Improve jumping between function/module definitions.

[NJichev]

In addition to #390, they can be improved to provide the following:

• ]m - go to start of next function definition.
• ]M - go to end of next function definition.
• [m go to start of previous function definition.
• [M go to end of previous function definition.
• ]] go to start of next module definition.
• ][ go to end of next module definition.
• [[ go to start of previous module definition.
• [] go to end of previous module definition.

Once again stolen from here

Those will add more control to you since the current behaviour is only:

• ]] - jump to beginning of next definition do/end
• [[ - jump to previous definition of a do/end

You lose control of jumping to the end part.
It might be pedantic to add different support for modules/functions.