Changes in this patch:

* Update the JSON-LD URL to HTTPS
* Update the list of JSON-LD keywords
* Make the JSON-LD parser less dependent on the JSON lexer implementation
* Add unit tests for the JSON-LD lexer

This includes:

* Testing valid literals
* Testing valid string escapes
* Testing that object keys are tokenized differently from string values

Related to pygments#1425

Included in this change:

* The JSON parser is rewritten
* The JSON bare object parser no longer requires additional code
* `get_tokens_unprocessed()` returns as much as it can to reduce yields
  (for example, side-by-side punctuation is not returned separately)
* The unit tests were updated
* Add unit tests based on Hypothesis test results

Related to pygments#1425

Tested on a 118MB JSON file. Memory consumption tops out at ~3GB before
this patch and drops to only ~2GB with this patch. These were the command
lines used:

python -m pygments -l json -f html -o .\new-code-classes.html .\jc-output.txt
python -m pygments -l json -f html -O "noclasses" -o .\new-code-styles.html .\jc-output.txt

…ting

For a 118MB JSON input file, this reduces memory consumption by ~500MB
and reduces formatting time by ~15 seconds.