Use escape_sequence for all escape types

[keidax]

The grammar has several types of escape sequence nodes:

• char_escape_sequence (only for chars)
• string_escape_sequence (everywhere else, not only for strings)
• ignored_backslash (used for "identity" escape sequences)

This PR renames all the above to escape_sequence. I've adjusted the highlighting as well, so escape_sequence is always highlighted in a color that contrasts with the surrounding content.

There are also line continuation backslashes, which are currently hidden nodes without any highlighting. These are unchanged. My intent here is that a line continuation looks different because it's not escaping a newline, it's obscuring a newline.

Highlighting improvements:

Before

After

[keidax]

@nobodywasishere probably needless to say, this is a breaking change for zed