Implement Long FHIRPath type

[johngrimes]

Spec link: https://build.fhir.org/ig/HL7/FHIRPath/#long

This change implements Long literals and also adds the Long type to our model.

Implement the Long FHIRPath type as defined in the specification to enable:

• Long literals: Support parsing of long values with 'L' suffix (e.g., 45L, -5L)
• Type system integration: Long as a first-class FHIRPath type alongside Integer, Decimal, etc.
• Range compliance: Support the full Long range (-2^63 to 2^63-1) as specified
• FHIR integer64: Map the FHIR integer64 type to Long in FHIRPath expressions
• Existing functions: Ensure Long values work correctly with currently implemented math, comparison, and logical functions
• Operator support: Support Long in arithmetic (+, -, *, /, mod), comparison (=, !=, <, >, <=, >=), and other relevant operators
• Type coercion: Handle Long operands in mixed-type expressions according to FHIRPath precedence rules
• Implicit conversion: Long → Integer

Out of Scope

• Conversion functions: toLong() and convertsToLong() functions (covered by Support all type conversion functions #2391)