Adds a new #with() tag to make it easier to embed and extend tags

[sowelipililimute]

Adds a new #with() tag to make it easier to work with extend and embed tags:

#with(parent):
  #(child)
#endwith

[0xTim]

Looks good on a first pass, will do a proper review when I'm at my desk

[0xTim]

@pontaoski is this ready for review? You mentioned on Discord you were doing more work

[sowelipililimute]

@pontaoski is this ready for review? You mentioned on Discord you were doing more work

Yea, this is ready for review. What I mentioned on Discord is basically me ADHDbraining and refactoring/rewriting most of Leaf's lexer and parser.

[0xTim]

Thanks!

[0xTim]

Original comment:

Commit One: add #with tag

This is similar to Go text/template's {{ with }} statement, in that it
allows overriding the context for the contained block.

Commit Two: add #with sugar for extend

This allows passing a context parameter to #extend, which will cause it
to desugar into a #with around the extended content, instead of just
the extended content itself.

[VaporBot]

These changes are now available in 1.7.0