[TS] Add AbortSignal to fetch template

[nulladdict]

This adds support for AbortSignal for Fetch template
I've mostly followed this PR for axios

One thing I'm concerned about is that it seems like AbortSignal typings were added in TypeScript 2.7, so I wonder if signal generation should be opt-in through some kind of setting

[nulladdict]

I've added conditional generation, depending on TypeScript version.
I wasn't sure if this should've been a separate setting, or computed from TypeScript version,
but I guess this should be fine, since generated signal is optional parameter

[RicoSuter]

I wasn't sure if this should've been a separate setting, or computed from TypeScript version,

Is this a feature which is supported in all browsers/fetch implementations?
Why is this dependent on the TS version?
If it's not supported in older versions/not in all fetch versions than we should probably put this behind a config..

[RicoSuter]

It's even experimental:

[nulladdict]

The original discussion about aborting fetch (1) (2) ended up using AbortSignal for Request, but I guess AbortSignal & AbortController are still considered experimental
Generally speaking, modern browsers that support fetch tend to also support this api, but additional polyfills might be necessary

I don't mind hiding this behind a config setting

Also, it's dependent on the TS version, because earlier versions didn't have types for AbortSignal & AbortController, but I think if we're putting this behind a setting this might not be relevant

[RicoSuter]

As its experimental and potentially a breaking change i think its better to put this behind a config (UseAbortSignal, default: false). Is that ok? Can you update the PR?

[nulladdict]

Sounds good to me, I'll update the PR

[RicoSuter]

@nulladdict can we merge this PR?

[nulladdict]

I think so, if there are no problems with liquid template.
I am not very experienced with it, it looks fine for me, but I might be missing something

[RicoSuter]

Lgtm, thanks for the PR.

[nulladdict]

No problem, should we also update the documentation for new UseAbortSignal config?
I'm not sure what (if anything) needs to be done to update the wiki

[RicoSuter]

[RicoSuter]

Just add a new group there and document it - that's probably fine. Thanks.