APC with standard ST terminator would break parser

[pda]

Buildkite uses ANSI Application Program Commands to insert timestamps into log streams. Technically APCs are terminated by “String Terminator” (ST; 0x9C), however in practice the non-standard 7-bit-clean “Bell” (BEL, 0x07) alternative tends to be used:

For historical reasons, Xterm can end the command with BEL as well as the standard ST
— https://en.wikipedia.org/wiki/ANSI_escape_code#OSC_(Operating_System_Command)_sequences

Our parser currently only supports BEL to terminate an APC. If an ST-terminated APC appeared in the log stream, our parser would miss everything until the next BEL or end-of-file.

This isn't an urgent issue, but maybe worth an easy fix one day.

Also described here as part of #99:

terminal-to-html/parser.go

Lines 141 to 157 in 384f24c

	// handleApplicationProgramCommand is called for each character consumed while
	// in MODE_APC, but does nothing until the APC is terminated with BEL (0x07).
	//
	// Technically an APC sequence is terminated by String Terminator (ST; 0x9C):
	// https://en.wikipedia.org/wiki/C0_and_C1_control_codes#C1_controls
	//
	// But:
	// > For historical reasons, Xterm can end the command with BEL as well as the standard ST
	// https://en.wikipedia.org/wiki/ANSI_escape_code#OSC_(Operating_System_Command)_sequences
	//
	// .. and this is how iTerm2 implements inline images:
	// > ESC ] 1337 ; key = value ^G
	// https://iterm2.com/documentation-images.html
	//
	// Buildkite's ansi timestamper does the same, and we don't _expect_ to be
	// seeing any other APCs that could be ST-terminated... 🤞🏼
	func (p *parser) handleApplicationProgramCommand(char rune) {