Entirely reworked text handling pipeline

This breaks the text pipeline into the following pieces:

- input parsing (split known bracketed phoneme sections out first)

- normalization (fix spacing, numbers, abbreviations, etc.)

- phonetization (do CMUdict lookup or g2p prediction)

- sequencing (convert the symbolic representation to integer
  representation)

Each "pipeline" can have a unique combination of operations for each of
these pipeline steps.

This will grow over time.

tts/text/pipeline_components/input_parsing/parser.py

@@ -0,0 +1,72 @@
+"""
+Parse raw text sequences into Arpabet and Grapheme sequences so that they can be 
+treated independently.
+
+Inspired by and including code from original Tacotron2 by Keith Ito.
+"""
+
+# pyright: strict
+
+import re
+from typing import List, Union, Any
+
+
+# Regular expression matching text enclosed in curly braces:
+_curly_re = re.compile(r'(.*?)\{(.+?)\}(.*)')
+
+
+class ArpabetSequence:
+    """
+    Denotes an Arpabet phoneme sequence corresponding to a single word.
+    """
+    def __init__(self, sequence: str):
+        self.sequence = sequence
+    def __eq__(self, other: Any) -> bool:
+        return isinstance(other, ArpabetSequence) and self.sequence == other.sequence
+    def __repr__(self):
+        return f"<ArpabetSequence: {self.sequence}>"
+
+
+class GraphemeSequence:
+    """
+    Denotes any string of non-Arpabet text, which may include multiple words,
+    spacing, and punctuation characters.
+    """
+    def __init__(self, sequence: str):
+        self.sequence = sequence
+    def __eq__(self, other: Any) -> bool:
+        return isinstance(other, GraphemeSequence) and self.sequence == other.sequence
+    def __repr__(self):
+        return f"<GraphemeSequence: {self.sequence}>"
+
+
+ParsedSequence = List[Union[ArpabetSequence, GraphemeSequence]]
+
+
+def parse_grapheme_and_arpabet_sequence(input_text: str) -> ParsedSequence:
+    """
+    Parse a string into a list of ArpabetSequence and GraphemeSequence so that each 
+    subsequence can be treated independently.
+    """
+    sequence : ParsedSequence = []
+
+    # Check for curly braces and treat their contents as ARPAbet:
+    while len(input_text):
+        input_text = input_text.strip()
+        m = _curly_re.match(input_text)
+        if not m:
+            sequence.append(GraphemeSequence(input_text))
+            break
+
+        before = m.group(1).strip()
+        bracketed = m.group(2).strip()
+        after = m.group(3).strip()
+
+        if before:
+            sequence.append(GraphemeSequence(before))
+        if bracketed:
+            sequence.append(ArpabetSequence(bracketed))
+
+        input_text = after
+
+    return sequence