Context

Adds a modern grammar-based expression parser. This is opt-in at this stage,
and can be used by instantiating a GrammarParser instead of the legacy
ShuntingYardParser.

The new parser fixes some long-standing issues with expression parsing, but
is still missing a few minor features that were previously supported by the legacy
parser, in particular:

• Implicit multiplication.
• Identifier naming is more restrictive, names must start with a letter or $,
  followed by any number of word characters (letter or digit). This is mostly
  relevant for user-provided identifiers (algorithmic functions and variables).

Features and fixes

• fix: Exception when define variable Number with character "e" #82
• fix: Suppress conversion into e function #61
• fix: Variables should resolve before keywords. #31
• feat: Add support for parsing constants, with initial support for piand e (Euler's constant) How to add pi , e and factorial to the expression? #26