[WIP] Big Restructuring: Canonical Assertion

Generate canonical assertion expression from JavaScript AST instead of searching/splitting JavaScript source code.

This feature enables us to instrument power-assert feature even if target assertion expression contains multiple lines. Therefore, Embedded AST is not required any more.

TODO
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- [x] Design again
- [x] Spike
- [x] Move escodegen to runtime dependency to generate canonical expression
- [x] Generate canonical expression instead of searched lines
- [x] Remove Embedded AST feature
- [x] Move esprima to development dependency since Embedded AST feature is gone
- [x] Ignore lineSeparator option
- [x] Ignore source option
- [x] Capture espath at assertion expression level
- [x] Use raw literal value for saved assertion content
- [x] More tests
- [ ] Refactoring
- [ ] Prepare for next minor release