multi-line string literals

TODO.md

@@ -253,25 +253,29 @@
   - parser-only change (`parse_branch_body` in `compiler/parser/parser.odin`): a brace-less
     body is just a 1-element statement slice, so the checker and codegen are unchanged
 
-17. add slice-by-range
-  - allow the use of a range in slice expressions:
-    ```
-    excl_range range :: 0..10
-    some_arr[excl_range] # slice by named exclusive range
-  
-    incl_range range :: 0..=10
-    some_arr[incl_range] # slice by named inclusive range
-    ```
+17. multi-line strings (implemented; see below)
+  - a multi-line string is an ordinary string literal under the hood: it lowers to the
+    same `.String` expr / `@[N;0]u8` type, so the checker, lowering, and codegen are
+    unchanged. only the lexer and parser change.
+  - the lexer (`compiler/lexer/lexer.odin`, `` case '`' ``) collapses consecutive
+    backtick-marked lines into one `Multiline_String` token; the trailing newline after
+    the last line stays a `.Newline` so it terminates the statement normally
+  - the parser (`decode_multiline_string` in `compiler/parser/parser.odin`) strips each
+    line's leading indentation and `` ` ``, takes the rest of the line raw (no escapes),
+    and joins lines with an implicit `\n` (no leading/trailing newline); an empty
+    `` ` `` yields a blank line
+  - the value may sit on the line after `=`/`::` (the existing post-operator
+    `skip_newlines` already allows this)
+  - `++` concatenation (the spec's "mixing" examples) is a separate, unimplemented
+    operator and is out of scope here
 
 18. add `defer` statement (inspired by zig)
 
-19. multi-line strings (see below)
+19. unions and tagged unions
 
-20. unions and tagged unions
+20. match statements with tagged unions payload unwrapping
 
-21. match statements with tagged unions payload unwrapping
-
-22. dynamic heap allocation
+21. dynamic heap allocation
   - see below for direction
   - notes below are too big in scope for a first pass and the language is not mature enough to support it yet
   - this first pass should focus on just basic heap allocation, so we have something to work with