Add escape sequence handling to the string literal lexer in compiler.c, translating
common escape sequences (\", \\, \n) into their literal character equivalents during
tokenization. Introduce a fixed-size buffer (MAX_STRING 1024) in the Parser struct to
accumulate the unescaped string content, along with a helper function addStringChar()
to append characters. The readString() function now loops over input characters,
breaking on an unescaped double quote, and processing backslash-prefixed escapes via
a switch statement. Remove the previous TODO placeholder comment and the elapsed time
computation simplification in benchmark/fib.lua. Add a new test file
test/string_escapes.wren that verifies the three supported escape sequences, and
remove the now-obsolete TODO about escapes from test/string_literals.wren.
