strings.textscanner: add rune mode support; add next_line() , read_until(), substr() funcs

[kbkpbot]

Introduce force_rune_mode params to the new(input string, config TextScannerConfig) and this will help scan runes instead of bytes of a input string.

@[params]
pub struct TextScannerConfig {
pub mut:
	force_rune_mode bool
}

// new returns a stack allocated instance of TextScanner.
pub fn new(input string, config TextScannerConfig) TextScanner

// next_line advances the scanner’s position to the start of
// the next line, and return the line.
// Returns true if successful, or false if the end of the input
// is reached.

pub fn (mut ss TextScanner) next_line() (string, bool) 

// read_until reads characters from the current scanning position
// until a delimiter (from the provided string `delimiters`) is encountered.
// The returned string includes all characters from the starting
// position up to (but ​not​ including) the first encountered
// delimiter. The scanner's position is advanced to the character
// immediately after the delimiter (or to the end of the input if
// no delimiter is found).

pub fn (mut ss TextScanner) read_until(delimiters string) !string

// substr return a sub string of input string from start to end.
pub fn (mut ss TextScanner) substr(start int, end int) string 

[huly-for-github]

Connected to Huly®: V_0.6-22278

[spytheman]

May be there is a misunderstanding - I do not want rune mode support, and if you do want it, you can just write your own scanner module, outside of vlib's strings. Supporting a rune mode only complicates and slows down the scanner.

My comment was just to point out, that you can pass a string for the separators, which is more compact.