Add BigDecimal parsing

[olbrich]

Add opt-in BigDecimal parsing for numeric literals in unit strings.

• Introduces RubyUnits::MissingDependencyError.
• Adds Configuration#use_bigdecimal (boolean, default: false). Enabling requires require 'bigdecimal'.
• Adds Unit.parse_number to centralize numeric coercion: returns BigDecimal when use_bigdecimal is enabled, otherwise Float.
• Updates parsing, conversion, and normalization code to use parse_number and normalize_to_i, and adds Unit#to_d so scalars may be BigDecimal and still normalize to Integer when mathematically integral.
• Adds specs in bigdecimal_parsing_spec.rb to validate parsing and normalization behavior when BigDecimal is enabled.

Misc: small RuboCop config and formatting changes.
Notes:

Important

BigDecimal parsing is opt-in; callers must require 'bigdecimal' before enabling.

Related to #372