Add string manipulation functions (trim, to_upper, to_lower, starts_with, ends_with)

[davidgaspardev]

Description

Add common string utility functions:

Function	Description
trim(src, dst)	Remove leading and trailing whitespace
to_upper(src, dst)	Convert all characters to uppercase
to_lower(src, dst)	Convert all characters to lowercase
starts_with(src, prefix)	Check if string starts with prefix
ends_with(src, suffix)	Check if string ends with suffix

References

• Standard: ASCII character table (uppercase A-Z: 0x41–0x5A, lowercase a-z: 0x61–0x7A)
• Standard C equivalents: toupper, tolower from <ctype.h> (locale-dependent)
• Wikipedia: ASCII
• Wikipedia: Trim (string)