feat: underscores in numeric literals

[latonis]

Work to implement and close #14

• modified pest grammar to allow for underscores following at least one number in a numeric literal, including floating point and those ending with the size suffixes
• added tests as well

Also tested with the following rule

rule test {
  condition:
    2_000 == 2000 and 100KB == 1_00KB and 0o12 == 1_0 and 0x2_1 == 0x21 and 0x31_1 == 7_8_5
}