Support class fields

Support for class fields (currently stage-3 proposal) is missing.

The class fields proposal is currently in Stage 3, and has a working implementation in Chrome, as well as being implemented in some other AST parsers (like babel).

I don't know at what stage features are normally added to esprima, but stage 3 looks like a solid stage to me.

Steps to reproduce

Parsing a class with a field just gives an Unexpected token error on the =-sign (expects a function with parentheses).

esprima.parse(`
class MyClass {
    myField = 1;
    #myPrivateField = 2;
}
`)

Expected output

Babel calls it a ClassProperty, with an key and value.

            {
              "type": "ClassProperty",
              "start": 15,
              "end": 28,
              "loc": { ... },
              "computed": false,
              "key": {
                "type": "Identifier",
                "start": 15,
                "end": 22,
                "loc": { ... },
                "name": "myField"
              },
              "static": false,
              "value": {
                "type": "Literal",
                "start": 25,
                "end": 28,
                "loc": { ... },
                "value": "1",
                "rawValue": "1",
                "raw": "'1'"
              }
            },

Private fields (starting wit #) are called a PrivateClassProperty.

Relevant references

https://github.com/tc39/proposal-class-fields

The issue provides a JavaScript class-fields example, expected ClassProperty and PrivateClassProperty AST shapes, and a reference to the TC39 proposal, but names no repository files, tests, or entry points. Start by locating the parser and existing class-member AST tests, then compare their conventions with the requested output. Done means the supplied public and private field examples parse into the expected AST structures.