Consolidate, refactor, and bugfix the lexer's handling of captures and skips (#1957)

- Do not error about local labels following keywords in skips or captures (fixes #1955)
- Do not incompletely attempt to handle line continuations in skips (fixes #1956)
- Rename `skipToLeadingIdentifier` to `skipToLeadingKeyword`, refactor to merge
  `skipToEOL` into it, and use it for both skips and captures

test/asm/line-continuation-if.asm

@@ -0,0 +1,34 @@
+MACRO m
+ENDM
+
+IF 0
+	m
+ENDC
+
+IF 0
+	m \
+ENDC
+
+IF 1
+	m
+ELSE
+	m
+ENDC
+
+IF 1
+	m
+ELSE
+	m \
+ENDC
+
+IF 1
+	m
+ELIF 0
+	m
+ENDC
+
+IF 1
+	m
+ELIF 0
+	m \
+ENDC