GeoCSS parser implementation

[allyoucanmap]

This PR introduces base write and read functionality for the parser.

It is still incomplete and limited in some parts that could be improved later. I'll try to summarize what's missing or needed below:

• missing tests to cover all functions and cases
• test and compare the results of the geostyler parser with the GeoSever translation GeoCSS to SLD to ensure correctness of rules
• nested filters are supported only with 3 levels with this structure [ '||', ... 0 [ '&&', ... 1 [ '||' or '&&' ... 2 ] ] ]. This is because I'm trying to follow the selector structure (see: GeoCSS filter doc). I'm aware it's possible to use nested parentheses like this [ type = 'e' and class = 'd' or ( position = 'a' or id = 'd') ] inside the selector but currently it's not supported (see flat-geo-css)
• missing not operator. It seems it's possible to use it only in come cases such attribute not in ( 'value' ) or attribute not like 'value' or attribute is not null (this part needs some investigation).
• missing StrMatchesFunctionFilter implementation

connected to issue geostyler/geostyler#1285

[chrismayer]

Thanks for providing your work and for your summary, what is missing @allyoucanmap. This helps a lot and makes it easier for others to continue on this. Very much appreciated 👍

[marcjansen]

Will someone continue to work on this? That'd be much appreciated

[allyoucanmap]

@marcjansen I'm sorry to come back so late to this PR. After the implementation of this branch we tried the parser by installing it as a github repository in our application and the functionality developed was fullfilling our requirements. Unfortunately I could not spend additional time to complete all the other features listed in the description and this seems a blocker for the merge of the PR in this repo. We currently need to create an npm package for this parser and for this reason we published it starting from our fork under a different scope to avoid conflicts with names (@geosolutions/geostyler-geocss-parser). We will update the testing part based on use cases that we will face while developing but we have not an estimates about this right now.