Set additional filetype options

[andrejtokarcik]

Defining setlocals that allow easy manipulation of keywords (e.g. by the star command) and comments (e.g. by the vim-commentary plugin).

[derekelkins]

I think the comments setting would annoy me so I'm not too keen on adding it. commentstring seems fine. iskeyword seems fine except that the 192-255 range seems to be based on the non-Windows default but for Windows the default in the extended ASCII range is 128-167,224-235. I'm not really sure how much this matters though.

[andrejtokarcik]

FWIW, I copied the comments setting from vim-polyglot's haskell.vim as Agda and Haskell use the same comment format.

I have just noticed more problems with this iskeyword value while testing with files from the standard library:

• The setting also includes round brackets which cannot be part of Agda identifiers. More generally, according to the wiki none of @.(){};_ can be a name part whereas the proposed iskeyword recognizes them as such. Underscore is an ambivalent case given that an agda-vim user might want to look for an infix/mixfix operator used in its prefix form specifically.
• Unicode characters are not handled predictably. They seem to be taken as identifiers when they are the last symbol on the line but otherwise Vim prefers to use the next token (after the Unicode char under cursor) fulfilling the iskeyword criteria.

Given all the issues I'm closing this pull request and will maybe open a new one once I have a more robust solution.