Encode chapter and variant separately in Share URL

[anubh-v]

Currently, when we create the Share URL for the Non-deterministic and Concurrent variants, we encode the chapter and variant information together in one string.

For example, if the playground was using Source 3 Non-Det, then then Share URL will look
like this https://source-academy.github.io/playground#chap=3_Non_Det&exec=1000&ext=NONE&prgrm=IYWwRgFAjAlA3EA

Note the string #chap=3_Non_Det in the URL.

Instead of this, we can encode the chapter and variant separately.
So, the url will look like: https://source-academy.github.io/playground#chap=3&exec=1000&ext=NONE&prgrm=IYWwRgFAjAlA3EA&variant=non-det

This PR reduces the steps needed to add a new language variant.
The Share URL will automatically support any language variants added into the sourceLangauges array in src/reducers/states.ts.

Note: The variant field in the Share URL is optional
If the URL does not have a variant, the playground will use the default variant.
This ensures the Share URL is backward compatible

[coveralls]

Pull Request Test Coverage Report for Build 4445

• 10 of 10 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 37.154%

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Totals
Change from base Build 4443:	-0.01%
Covered Lines:	2800
Relevant Lines:	6748

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💛 - Coveralls

[anubh-v]

If the URL has a chapter - variant combination that is not supported, then the variant will be set to default.

One example is if the URL has chapter = 4, but variant = non-det

[martin-henz]

If the URL has a chapter - variant combination that is not supported, then the variant will be set to default.

One example is if the URL has chapter = 4, but variant = non-det

Not sure if that's the right choice. I see four options:
(1) change variant to "default"
(2) increase chapter until a matching variant exists
(3) decrease chapter until a matching variant exists
(4) reject program

I think in any case (except perhaps 2) we should give a warning.

A clear and simple solution would be simply (4) with an error message.

If you want to be fancy, you could:

• Follow (2), if not possible:
• Follow (3) and give warning, if not possible:
• Follow (1) with a warning, if not possible:
• Follow (4) with an error message.

[martin-henz]

Regarding the question what to do if we dont have a chapter with this variant:

If the URL has a chapter - variant combination that is not supported, then the variant will be set to default.

One example is if the URL has chapter = 4, but variant = non-det

Not sure if that's the right choice. I see four options:
(1) change variant to "default"
(2) increase chapter until a matching variant exists
(3) decrease chapter until a matching variant exists
(4) reject program

I think in any case (except perhaps 2) we should give a warning.

A clear and simple solution would be simply (4) with an error message.

If you want to be fancy, you could:

• Follow (2), if not possible:
• Follow (3) and give warning, if not possible:
• Follow (1) with a warning, if not possible:
• Follow (4) with an error message.