fix(treesitter): no injection highlighting on last line #36951

Problem:
If the last visible line in a window is not fully displayed, this line
may not get injection highlighting. This happens because line('w$')
actually means the last *completely displayed* line.

Solution:
Use line('w$') + 1 for the botline.

This reverts 4244a96774
"test: fix failing lsp/utils_spec #36609",
which changed the test based on the wrong behavior.

test/functional/plugin/lsp/utils_spec.lua

@@ -357,7 +357,7 @@ describe('vim.lsp.util', function()
     screen:expect([[
       ^                                                     |
       ┌─────────┐{1:                                          }|
-      │{101:local foo}│{1:                                          }|
+      │{100:local}{101: }{102:foo}│{1:                                          }|
       └─────────┘{1:                                          }|
       {1:~                                                    }|*9
                                                            |
@@ -384,7 +384,7 @@ describe('vim.lsp.util', function()
     screen:expect([[
       ^                                                     |
       ┌─────────┐{1:                                          }|
-      │{101:local foo}│{1:                                          }|
+      │{100:local}{101: }{102:foo}│{1:                                          }|
       └─────────┘{1:                                          }|
       {1:~                                                    }|*9
                                                            |