Add "Open arXiv" action to cite context menu

[postylem]

Makes a new item to the VimtexContextMenu, so when doing <localleader>la on a citation, you get the option to open it on arXiv.org if the citation is an arXiv preprint.

This PR is just something I had done in my own branch so I could use it. I thought I'd make it available to others this way. I hope I'm following the correct procedure here (should I have suggested it in an issue first? I don't know the etiquette. Should I have created some tests too? It worked for me with the following citations).

@article{0801.1144,
	author = {John Kitchin and Karsten Reuter and Matthias Scheffler},
	doi = {10.1103/PhysRevB.77.075437},
	eprint = {arXiv:0801.1144},
	title = {Alloy surface segregation in reactive environments: A first-principles atomistic thermodynamics study of Ag3Pd(111) in oxygen atmospheres},
	year = {2008}}

@misc{wilcox.e:2021,
	archiveprefix = {arXiv},
	author = {Ethan Gotlieb Wilcox and Pranali Vani and Roger P. Levy},
	eprint = {2106.03232},
	primaryclass = {cs.CL},
	title = {A Targeted Assessment of Incremental Processing in Neural Language Models and Humans},
	year = {2021}}

[lervag]

Makes a new item to the VimtexContextMenu, so when doing <localleader>la on a citation, you get the option to open it on arXiv.org if the citation is an arXiv preprint.

Thanks, that's useful!

This PR is just something I had done in my own branch so I could use it. I thought I'd make it available to others this way. I hope I'm following the correct procedure here (should I have suggested it in an issue first? I don't know the etiquette. Should I have created some tests too? It worked for me with the following citations).

This works fine! From my point of view, the only important procedure is that the communication of ideas and issues are clear, and by submitting a PR like this you make it perfectly clear what you want and you even propose a solution. It doesn't get much better than that. :)

I did make some changes before I merged:

• I use 2 spaces indents and 80 columns maximum. I sometimes break the line width rule, but only if there is a good reason for it.
• I added the example entries and a couple of simple tests.

[postylem]

Ok I see thanks for the tips. I’ll obey the style if I make any further PRs. This is my first foray into vimscript so to be honest I wasn’t sure how to break long lines.

[lervag]

No problem :)