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

test/asm/local-after-keyword.asm

@@ -0,0 +1,20 @@
+section "test", rom0
+
+if 0
+	section.local "oops"
+else
+	println "*sips coffee*"
+endc
+
+rept 0
+	assert.local "lol"
+endr
+rept 1
+	println "this is fine"
+endr
+
+macro m
+	db.local 42
+endm
+
+db.local 123

test/asm/local-after-keyword.err

@@ -0,0 +1,5 @@
+error: Identifier "db.local" begins with a keyword; did you mean to put a space between them?
+    at local-after-keyword.asm(20)
+error: syntax error, unexpected number
+    at local-after-keyword.asm(20)
+Assembly aborted with 2 errors

test/asm/local-after-keyword.out

@@ -0,0 +1,2 @@
+*sips coffee*
+this is fine