fix: skip only spaces at line start in Verso docstrings (#14191)

This PR fixes an issue where escaped content at the start of a line in a
valid block-opening positon in Verso content was skipped as if it were
whitespace.

Author: david-christiansen

src/Lean/DocString/Parser.lean

@@ -100,8 +100,6 @@ partial def takeUntilEscFn (p : Char → Bool) : ParserFn := fun c s =>
   else if p (c.get' i h) then s
   else takeUntilEscFn p c (s.next' c i h)
 
-partial def takeWhileEscFn (p : Char → Bool) : ParserFn := takeUntilEscFn (not ∘ p)
-
 /--
 Parses as `p`, but discards the result.
 -/
@@ -382,7 +380,7 @@ Parses block opener prefixes. At the beginning of the line, if this parser succe
 block is beginning.
 -/
 public def blockOpener := atomicFn <|
-  takeWhileEscFn (· == ' ') >>
+  eatSpaces >>
   (atomicFn ((bullet >> chFn ' ')) <|> -- Unordered list
    atomicFn ((numbering >> chFn ' ')) <|> -- Ordered list
    atomicFn (strFn ": ") <|> -- Description list item

tests/docparse/blockOpener_0004.txt

@@ -0,0 +1 @@
+\[> x

tests/docparse/blockOpener_0004.txt.out.expected

@@ -0,0 +1,4 @@
+Failure @0 (⟨1, 0⟩): '>'; expected '%%%' or ': '
+Final stack:
+ empty
+Remaining: "\\[> x"

tests/docparse/blockOpener_0005.txt

@@ -0,0 +1 @@
+\*> x

tests/docparse/blockOpener_0005.txt.out.expected

@@ -0,0 +1,4 @@
+Failure @0 (⟨1, 0⟩): '>'; expected '%%%' or ': '
+Final stack:
+ empty
+Remaining: "\\*> x"

tests/docparse/blockOpener_0006.txt

@@ -0,0 +1 @@
+  \[> x

tests/docparse/blockOpener_0006.txt.out.expected

@@ -0,0 +1,4 @@
+Failure @0 (⟨1, 0⟩): '>'; expected '%%%' or ': '
+Final stack:
+ empty
+Remaining: "  \\[> x"

tests/docparse/blockOpener_0007.txt

@@ -0,0 +1 @@
+  > x

tests/docparse/blockOpener_0007.txt.out.expected

@@ -0,0 +1,4 @@
+Success! Final stack:
+ empty
+Remaining:
+" x"

tests/docparse/blocks_0055.txt

@@ -0,0 +1 @@
+\[> x