Add support for scientific notation with 'e' and 'E'

[kohlerdominik]

This PR fixes issue #96, describing that scientific notation (e.g., 1e-7, 1e+7) is incorrectly parsed as an arithmetic operation involving the Euler constant e.

Changes

1. Lexer Look-ahead Logic

Modified src/lexer.ts to implement a look-ahead check. When the lexer encounters e or E preceded by a number, it now checks for:

• A trailing number (e.g., 1e7)
• A sign followed by a number (e.g., 1e-7 or 1e+7)

If these patterns match, they are collapsed into the NUMBER token instead of being treated as constants or operators.

2. Capital E Support

Added E as a valid token in src/token.ts for parity with e, allowing 1E-7 to be parsed correctly.

3. Euler Constant Preservation

Standalone usage of the constant e remains unaffected. The lexer only triggers the scientific notation logic if the e is immediately preceded by a numeric value.

4. New Test Suite

Added test/scientific-notation.js with 11 test cases.